Do all audio mixing in the AudioServer

scene/2d/audio_stream_player_2d.cpp

@@ -33,125 +33,17 @@
 #include "scene/2d/area_2d.h"
 #include "scene/main/window.h"
 
-void AudioStreamPlayer2D::_mix_audio() {
-	if (!stream_playback.is_valid() || !active.is_set() ||
-			(stream_paused && !stream_paused_fade_out)) {
-		return;
-	}
-
-	if (setseek.get() >= 0.0) {
-		stream_playback->start(setseek.get());
-		setseek.set(-1.0); //reset seek
-	}
-
-	//get data
-	AudioFrame *buffer = mix_buffer.ptrw();
-	int buffer_size = mix_buffer.size();
-
-	if (stream_paused_fade_out) {
-		// Short fadeout ramp
-		buffer_size = MIN(buffer_size, 128);
-	}
-
-	stream_playback->mix(buffer, pitch_scale, buffer_size);
-
-	//write all outputs
-	int oc = output_count.get();
-	for (int i = 0; i < oc; i++) {
-		Output current = outputs[i];
-
-		//see if current output exists, to keep volume ramp
-		bool found = false;
-		for (int j = i; j < prev_output_count; j++) {
-			if (prev_outputs[j].viewport == current.viewport) {
-				if (j != i) {
-					SWAP(prev_outputs[j], prev_outputs[i]);
-				}
-				found = true;
-				break;
-			}
-		}
-
-		if (!found) {
-			//create new if was not used before
-			if (prev_output_count < MAX_OUTPUTS) {
-				prev_outputs[prev_output_count] = prev_outputs[i]; //may be owned by another viewport
-				prev_output_count++;
-			}
-			prev_outputs[i] = current;
-		}
-
-		//mix!
-		AudioFrame target_volume = stream_paused_fade_out ? AudioFrame(0.f, 0.f) : current.vol;
-		AudioFrame vol_prev = stream_paused_fade_in ? AudioFrame(0.f, 0.f) : prev_outputs[i].vol;
-		AudioFrame vol_inc = (target_volume - vol_prev) / float(buffer_size);
-		AudioFrame vol = vol_prev;
-
-		int cc = AudioServer::get_singleton()->get_channel_count();
-
-		if (cc == 1) {
-			if (!AudioServer::get_singleton()->thread_has_channel_mix_buffer(current.bus_index, 0)) {
-				continue; //may have been removed
-			}
-
-			AudioFrame *target = AudioServer::get_singleton()->thread_get_channel_mix_buffer(current.bus_index, 0);
-
-			for (int j = 0; j < buffer_size; j++) {
-				target[j] += buffer[j] * vol;
-				vol += vol_inc;
-			}
-
-		} else {
-			AudioFrame *targets[4];
-			bool valid = true;
-
-			for (int k = 0; k < cc; k++) {
-				if (!AudioServer::get_singleton()->thread_has_channel_mix_buffer(current.bus_index, k)) {
-					valid = false; //may have been removed
-					break;
-				}
-
-				targets[k] = AudioServer::get_singleton()->thread_get_channel_mix_buffer(current.bus_index, k);
-			}
-
-			if (!valid) {
-				continue;
-			}
-
-			for (int j = 0; j < buffer_size; j++) {
-				AudioFrame frame = buffer[j] * vol;
-				for (int k = 0; k < cc; k++) {
-					targets[k][j] += frame;
-				}
-				vol += vol_inc;
-			}
-		}
-
-		prev_outputs[i] = current;
-	}
-
-	prev_output_count = oc;
-
-	//stream is no longer active, disable this.
-	if (!stream_playback->is_playing()) {
-		active.clear();
-	}
-
-	output_ready.clear();
-	stream_paused_fade_in = false;
-	stream_paused_fade_out = false;
-}
-
 void AudioStreamPlayer2D::_notification(int p_what) {
 	if (p_what == NOTIFICATION_ENTER_TREE) {
-		AudioServer::get_singleton()->add_callback(_mix_audios, this);
+		AudioServer::get_singleton()->add_listener_changed_callback(_listener_changed_cb, this);
 		if (autoplay && !Engine::get_singleton()->is_editor_hint()) {
 			play();
 		}
 	}
 
 	if (p_what == NOTIFICATION_EXIT_TREE) {
-		AudioServer::get_singleton()->remove_callback(_mix_audios, this);
+		stop();
+		AudioServer::get_singleton()->remove_listener_changed_callback(_listener_changed_cb, this);
 	}
 
 	if (p_what == NOTIFICATION_PAUSED) {
@@ -168,109 +60,120 @@ void AudioStreamPlayer2D::_notification(int p_what) {
 	if (p_what == NOTIFICATION_INTERNAL_PHYSICS_PROCESS) {
 		//update anything related to position first, if possible of course
 
-		if (!output_ready.is_set()) {
-			Ref<World2D> world_2d = get_world_2d();
-			ERR_FAIL_COND(world_2d.is_null());
+		if (!stream_playback.is_valid()) {
+			return;
+		}
+		if (setplay.get() >= 0 || (active.is_set() && last_mix_count != AudioServer::get_singleton()->get_mix_count())) {
+			_update_panning();
+			if (setplay.get() >= 0) {
+				active.set();
+				AudioServer::get_singleton()->start_playback_stream(stream_playback, _get_actual_bus(), volume_vector, setplay.get());
+				setplay.set(-1);
+			}
+		}
+
+		// Stop playing if no longer active.
+		if (active.is_set() && !AudioServer::get_singleton()->is_playback_active(stream_playback)) {
+			active.clear();
+			set_physics_process_internal(false);
+			emit_signal(SNAME("finished"));
+		}
+	}
+}
 
-			int new_output_count = 0;
+StringName AudioStreamPlayer2D::_get_actual_bus() {
+	if (!stream_playback.is_valid()) {
+		return SNAME("Master");
+	}
 
-			Vector2 global_pos = get_global_position();
+	Vector2 global_pos = get_global_position();
 
-			int bus_index = AudioServer::get_singleton()->thread_find_bus_index(bus);
+	//check if any area is diverting sound into a bus
+	Ref<World2D> world_2d = get_world_2d();
+	ERR_FAIL_COND_V(world_2d.is_null(), SNAME("Master"));
 
-			//check if any area is diverting sound into a bus
+	PhysicsDirectSpaceState2D *space_state = PhysicsServer2D::get_singleton()->space_get_direct_state(world_2d->get_space());
+	PhysicsDirectSpaceState2D::ShapeResult sr[MAX_INTERSECT_AREAS];
 
-			PhysicsDirectSpaceState2D *space_state = PhysicsServer2D::get_singleton()->space_get_direct_state(world_2d->get_space());
+	int areas = space_state->intersect_point(global_pos, sr, MAX_INTERSECT_AREAS, Set<RID>(), area_mask, false, true);
 
-			PhysicsDirectSpaceState2D::ShapeResult sr[MAX_INTERSECT_AREAS];
+	for (int i = 0; i < areas; i++) {
+		Area2D *area2d = Object::cast_to<Area2D>(sr[i].collider);
+		if (!area2d) {
+			continue;
+		}
 
-			int areas = space_state->intersect_point(global_pos, sr, MAX_INTERSECT_AREAS, Set<RID>(), area_mask, false, true);
+		if (!area2d->is_overriding_audio_bus()) {
+			continue;
+		}
 
-			for (int i = 0; i < areas; i++) {
-				Area2D *area2d = Object::cast_to<Area2D>(sr[i].collider);
-				if (!area2d) {
-					continue;
-				}
+		return area2d->get_audio_bus_name();
+	}
+	return default_bus;
+}
 
-				if (!area2d->is_overriding_audio_bus()) {
-					continue;
-				}
+void AudioStreamPlayer2D::_update_panning() {
+	if (!stream_playback.is_valid()) {
+		return;
+	}
 
-				StringName bus_name = area2d->get_audio_bus_name();
-				bus_index = AudioServer::get_singleton()->thread_find_bus_index(bus_name);
-				break;
-			}
+	last_mix_count = AudioServer::get_singleton()->get_mix_count();
 
-			const Set<Viewport *> viewports = world_2d->get_viewports();
+	Ref<World2D> world_2d = get_world_2d();
+	ERR_FAIL_COND(world_2d.is_null());
 
-			for (Set<Viewport *>::Element *E = viewports.front(); E; E = E->next()) {
-				Viewport *vp = E->get();
-				if (vp->is_audio_listener_2d()) {
-					//compute matrix to convert to screen
-					Transform2D to_screen = vp->get_global_canvas_transform() * vp->get_canvas_transform();
-					Vector2 screen_size = vp->get_visible_rect().size;
+	Vector2 global_pos = get_global_position();
 
-					//screen in global is used for attenuation
-					Vector2 screen_in_global = to_screen.affine_inverse().xform(screen_size * 0.5);
+	Set<Viewport *> viewports = world_2d->get_viewports();
+	viewports.insert(get_viewport()); // TODO: This is a mediocre workaround for #50958. Remove when that bug is fixed!
 
-					float dist = global_pos.distance_to(screen_in_global); //distance to screen center
+	volume_vector.resize(4);
+	volume_vector.write[0] = AudioFrame(0, 0);
+	volume_vector.write[1] = AudioFrame(0, 0);
+	volume_vector.write[2] = AudioFrame(0, 0);
+	volume_vector.write[3] = AudioFrame(0, 0);
 
-					if (dist > max_distance) {
-						continue; //can't hear this sound in this viewport
-					}
+	for (Viewport *vp : viewports) {
+		if (!vp->is_audio_listener_2d()) {
+			continue;
+		}
+		//compute matrix to convert to screen
+		Transform2D to_screen = vp->get_global_canvas_transform() * vp->get_canvas_transform();
+		Vector2 screen_size = vp->get_visible_rect().size;
 
-					float multiplier = Math::pow(1.0f - dist / max_distance, attenuation);
-					multiplier *= Math::db2linear(volume_db); //also apply player volume!
+		//screen in global is used for attenuation
+		Vector2 screen_in_global = to_screen.affine_inverse().xform(screen_size * 0.5);
 
-					//point in screen is used for panning
-					Vector2 point_in_screen = to_screen.xform(global_pos);
+		float dist = global_pos.distance_to(screen_in_global); //distance to screen center
 
-					float pan = CLAMP(point_in_screen.x / screen_size.width, 0.0, 1.0);
+		if (dist > max_distance) {
+			continue; //can't hear this sound in this viewport
+		}
 
-					float l = 1.0 - pan;
-					float r = pan;
+		float multiplier = Math::pow(1.0f - dist / max_distance, attenuation);
+		multiplier *= Math::db2linear(volume_db); //also apply player volume!
 
-					outputs[new_output_count].vol = AudioFrame(l, r) * multiplier;
-					outputs[new_output_count].bus_index = bus_index;
-					outputs[new_output_count].viewport = vp; //keep pointer only for reference
-					new_output_count++;
-					if (new_output_count == MAX_OUTPUTS) {
-						break;
-					}
-				}
-			}
+		//point in screen is used for panning
+		Vector2 point_in_screen = to_screen.xform(global_pos);
 
-			output_count.set(new_output_count);
-			output_ready.set();
-		}
+		float pan = CLAMP(point_in_screen.x / screen_size.width, 0.0, 1.0);
 
-		//start playing if requested
-		if (setplay.get() >= 0.0) {
-			setseek.set(setplay.get());
-			active.set();
-			setplay.set(-1);
-		}
+		float l = 1.0 - pan;
+		float r = pan;
 
-		//stop playing if no longer active
-		if (!active.is_set()) {
-			set_physics_process_internal(false);
-			emit_signal(SNAME("finished"));
-		}
+		volume_vector.write[0] = AudioFrame(l, r) * multiplier;
 	}
+
+	AudioServer::get_singleton()->set_playback_bus_exclusive(stream_playback, _get_actual_bus(), volume_vector);
 }
 
 void AudioStreamPlayer2D::set_stream(Ref<AudioStream> p_stream) {
-	AudioServer::get_singleton()->lock();
-
-	mix_buffer.resize(AudioServer::get_singleton()->thread_get_mix_buffer_size());
-
 	if (stream_playback.is_valid()) {
-		stream_playback.unref();
-		stream.unref();
-		active.clear();
-		setseek.set(-1);
+		stop();
 	}
 
+	stream_playback.unref();
+	stream.unref();
 	if (p_stream.is_valid()) {
 		stream_playback = p_stream->instance_playback();
 		if (stream_playback.is_valid()) {
@@ -280,8 +183,6 @@ void AudioStreamPlayer2D::set_stream(Ref<AudioStream> p_stream) {
 		}
 	}
 
-	AudioServer::get_singleton()->unlock();
-
 	if (p_stream.is_valid() && stream_playback.is_null()) {
 		stream.unref();
 	}
@@ -302,6 +203,9 @@ float AudioStreamPlayer2D::get_volume_db() const {
 void AudioStreamPlayer2D::set_pitch_scale(float p_pitch_scale) {
 	ERR_FAIL_COND(p_pitch_scale <= 0.0);
 	pitch_scale = p_pitch_scale;
+	if (stream_playback.is_valid()) {
+		AudioServer::get_singleton()->set_playback_pitch_scale(stream_playback, p_pitch_scale);
+	}
 }
 
 float AudioStreamPlayer2D::get_pitch_scale() const {
@@ -309,27 +213,26 @@ float AudioStreamPlayer2D::get_pitch_scale() const {
 }
 
 void AudioStreamPlayer2D::play(float p_from_pos) {
-	if (!is_playing()) {
-		// Reset the prev_output_count if the stream is stopped
-		prev_output_count = 0;
+	stop();
+	if (stream.is_valid()) {
+		stream_playback = stream->instance_playback();
 	}
-
 	if (stream_playback.is_valid()) {
 		setplay.set(p_from_pos);
-		output_ready.clear();
 		set_physics_process_internal(true);
 	}
 }
 
 void AudioStreamPlayer2D::seek(float p_seconds) {
-	if (stream_playback.is_valid()) {
-		setseek.set(p_seconds);
+	if (stream_playback.is_valid() && active.is_set()) {
+		play(p_seconds);
 	}
 }
 
 void AudioStreamPlayer2D::stop() {
 	if (stream_playback.is_valid()) {
 		active.clear();
+		AudioServer::get_singleton()->stop_playback_stream(stream_playback);
 		set_physics_process_internal(false);
 		setplay.set(-1);
 	}
@@ -337,7 +240,7 @@ void AudioStreamPlayer2D::stop() {
 
 bool AudioStreamPlayer2D::is_playing() const {
 	if (stream_playback.is_valid()) {
-		return active.is_set() || setplay.get() >= 0;
+		return AudioServer::get_singleton()->is_playback_active(stream_playback);
 	}
 
 	return false;
@@ -345,30 +248,23 @@ bool AudioStreamPlayer2D::is_playing() const {
 
 float AudioStreamPlayer2D::get_playback_position() {
 	if (stream_playback.is_valid()) {
-		float ss = setseek.get();
-		if (ss >= 0.0) {
-			return ss;
-		}
-		return stream_playback->get_playback_position();
+		return AudioServer::get_singleton()->get_playback_position(stream_playback);
 	}
 
 	return 0;
 }
 
 void AudioStreamPlayer2D::set_bus(const StringName &p_bus) {
-	//if audio is active, must lock this
-	AudioServer::get_singleton()->lock();
-	bus = p_bus;
-	AudioServer::get_singleton()->unlock();
+	default_bus = p_bus; // This will be pushed to the audio server during the next physics timestep, which is fast enough.
 }
 
 StringName AudioStreamPlayer2D::get_bus() const {
 	for (int i = 0; i < AudioServer::get_singleton()->get_bus_count(); i++) {
-		if (AudioServer::get_singleton()->get_bus_name(i) == bus) {
-			return bus;
+		if (AudioServer::get_singleton()->get_bus_name(i) == default_bus) {
+			return default_bus;
 		}
 	}
-	return "Master";
+	return SNAME("Master");
 }
 
 void AudioStreamPlayer2D::set_autoplay(bool p_enable) {
@@ -388,7 +284,11 @@ void AudioStreamPlayer2D::_set_playing(bool p_enable) {
 }
 
 bool AudioStreamPlayer2D::_is_active() const {
-	return active.is_set();
+	if (stream_playback.is_valid()) {
+		// TODO make sure this doesn't change any behavior w.r.t. pauses. Is a paused stream active?
+		return AudioServer::get_singleton()->is_playback_active(stream_playback);
+	}
+	return false;
 }
 
 void AudioStreamPlayer2D::_validate_property(PropertyInfo &property) const {
@@ -436,15 +336,17 @@ uint32_t AudioStreamPlayer2D::get_area_mask() const {
 }
 
 void AudioStreamPlayer2D::set_stream_paused(bool p_pause) {
-	if (p_pause != stream_paused) {
-		stream_paused = p_pause;
-		stream_paused_fade_in = !p_pause;
-		stream_paused_fade_out = p_pause;
+	// TODO this does not have perfect recall, fix that maybe? If the stream isn't set, we can't persist this bool.
+	if (stream_playback.is_valid()) {
+		AudioServer::get_singleton()->set_playback_paused(stream_playback, p_pause);
 	}
 }
 
 bool AudioStreamPlayer2D::get_stream_paused() const {
-	return stream_paused;
+	if (stream_playback.is_valid()) {
+		return AudioServer::get_singleton()->is_playback_paused(stream_playback);
+	}
+	return false;
 }
 
 Ref<AudioStreamPlayback> AudioStreamPlayer2D::get_stream_playback() {

scene/2d/audio_stream_player_2d.h

@@ -51,38 +51,30 @@ private:
 		Viewport *viewport = nullptr; //pointer only used for reference to previous mix
 	};
 
-	Output outputs[MAX_OUTPUTS];
-	SafeNumeric<int> output_count;
-	SafeFlag output_ready;
-
-	//these are used by audio thread to have a reference of previous volumes (for ramping volume and avoiding clicks)
-	Output prev_outputs[MAX_OUTPUTS];
-	int prev_output_count = 0;
-
 	Ref<AudioStreamPlayback> stream_playback;
 	Ref<AudioStream> stream;
-	Vector<AudioFrame> mix_buffer;
 
-	SafeNumeric<float> setseek{ -1.0 };
 	SafeFlag active;
 	SafeNumeric<float> setplay{ -1.0 };
 
+	Vector<AudioFrame> volume_vector;
+
+	uint64_t last_mix_count = -1;
+
 	float volume_db = 0.0;
 	float pitch_scale = 1.0;
 	bool autoplay = false;
-	bool stream_paused = false;
-	bool stream_paused_fade_in = false;
-	bool stream_paused_fade_out = false;
-	StringName bus;
-
-	void _mix_audio();
-	static void _mix_audios(void *self) { reinterpret_cast<AudioStreamPlayer2D *>(self)->_mix_audio(); }
+	StringName default_bus = "Master";
 
 	void _set_playing(bool p_enable);
 	bool _is_active() const;
 
+	StringName _get_actual_bus();
+	void _update_panning();
 	void _bus_layout_changed();
 
+	static void _listener_changed_cb(void *self) { reinterpret_cast<AudioStreamPlayer2D *>(self)->_update_panning(); }
+
 	uint32_t area_mask = 1;
 
 	float max_distance = 2000.0;

scene/3d/audio_stream_player_3d.cpp

@@ -95,7 +95,7 @@ static const Vector3 speaker_directions[7] = {
 	Vector3(1.0, 0.0, 0.0).normalized(), // side-right
 };
 
-void AudioStreamPlayer3D::_calc_output_vol(const Vector3 &source_dir, real_t tightness, AudioStreamPlayer3D::Output &output) {
+void AudioStreamPlayer3D::_calc_output_vol(const Vector3 &source_dir, real_t tightness, Vector<AudioFrame> &output) {
 	unsigned int speaker_count = 0; // only main speakers (no LFE)
 	switch (AudioServer::get_singleton()->get_speaker_mode()) {
 		case AudioServer::SPEAKER_MODE_STEREO:
@@ -118,182 +118,94 @@ void AudioStreamPlayer3D::_calc_output_vol(const Vector3 &source_dir, real_t tig
 
 	switch (AudioServer::get_singleton()->get_speaker_mode()) {
 		case AudioServer::SPEAKER_SURROUND_71:
-			output.vol[3].l = volumes[5]; // side-left
-			output.vol[3].r = volumes[6]; // side-right
+			output.write[3].l = volumes[5]; // side-left
+			output.write[3].r = volumes[6]; // side-right
 			[[fallthrough]];
 		case AudioServer::SPEAKER_SURROUND_51:
-			output.vol[2].l = volumes[3]; // rear-left
-			output.vol[2].r = volumes[4]; // rear-right
+			output.write[2].l = volumes[3]; // rear-left
+			output.write[2].r = volumes[4]; // rear-right
 			[[fallthrough]];
 		case AudioServer::SPEAKER_SURROUND_31:
-			output.vol[1].r = 1.0; // LFE - always full power
-			output.vol[1].l = volumes[2]; // center
+			output.write[1].r = 1.0; // LFE - always full power
+			output.write[1].l = volumes[2]; // center
 			[[fallthrough]];
 		case AudioServer::SPEAKER_MODE_STEREO:
-			output.vol[0].r = volumes[1]; // front-right
-			output.vol[0].l = volumes[0]; // front-left
+			output.write[0].r = volumes[1]; // front-right
+			output.write[0].l = volumes[0]; // front-left
 			break;
 	}
 }
 
-void AudioStreamPlayer3D::_mix_audio() {
-	if (!stream_playback.is_valid() || !active.is_set() ||
-			(stream_paused && !stream_paused_fade_out)) {
-		return;
-	}
+void AudioStreamPlayer3D::_calc_reverb_vol(Area3D *area, Vector3 listener_area_pos, Vector<AudioFrame> direct_path_vol, Vector<AudioFrame> &reverb_vol) {
+	reverb_vol.resize(4);
+	reverb_vol.write[0] = AudioFrame(0, 0);
+	reverb_vol.write[1] = AudioFrame(0, 0);
+	reverb_vol.write[2] = AudioFrame(0, 0);
+	reverb_vol.write[3] = AudioFrame(0, 0);
 
-	bool started = false;
-	if (setseek.get() >= 0.0) {
-		stream_playback->start(setseek.get());
-		setseek.set(-1.0); //reset seek
-		started = true;
-	}
+	float uniformity = area->get_reverb_uniformity();
+	float area_send = area->get_reverb_amount();
 
-	//get data
-	AudioFrame *buffer = mix_buffer.ptrw();
-	int buffer_size = mix_buffer.size();
+	if (uniformity > 0.0) {
+		float distance = listener_area_pos.length();
+		float attenuation = Math::db2linear(_get_attenuation_db(distance));
 
-	if (stream_paused_fade_out) {
-		// Short fadeout ramp
-		buffer_size = MIN(buffer_size, 128);
-	}
+		// Determine the fraction of sound that would come from each speaker if they were all driven uniformly.
+		float center_val[3] = { 0.5f, 0.25f, 0.16666f };
+		int channel_count = AudioServer::get_singleton()->get_channel_count();
+		AudioFrame center_frame(center_val[channel_count - 1], center_val[channel_count - 1]);
 
-	// Mix if we're not paused or we're fading out
-	if ((output_count.get() > 0 || out_of_range_mode == OUT_OF_RANGE_MIX)) {
-		float output_pitch_scale = 0.0;
-		if (output_count.get()) {
-			//used for doppler, not realistic but good enough
-			for (int i = 0; i < output_count.get(); i++) {
-				output_pitch_scale += outputs[i].pitch_scale;
-			}
-			output_pitch_scale /= float(output_count.get());
-		} else {
-			output_pitch_scale = 1.0;
-		}
+		if (attenuation < 1.0) {
+			//pan the uniform sound
+			Vector3 rev_pos = listener_area_pos;
+			rev_pos.y = 0;
+			rev_pos.normalize();
 
-		stream_playback->mix(buffer, pitch_scale * output_pitch_scale, buffer_size);
-	}
-
-	//write all outputs
-	for (int i = 0; i < output_count.get(); i++) {
-		Output current = outputs[i];
-
-		//see if current output exists, to keep volume ramp
-		bool found = false;
-		for (int j = i; j < prev_output_count; j++) {
-			if (prev_outputs[j].viewport == current.viewport) {
-				if (j != i) {
-					SWAP(prev_outputs[j], prev_outputs[i]);
-				}
-				found = true;
-				break;
+			if (channel_count >= 1) {
+				// Stereo pair
+				float c = rev_pos.x * 0.5 + 0.5;
+				reverb_vol.write[0].l = 1.0 - c;
+				reverb_vol.write[0].r = c;
 			}
-		}
 
-		bool interpolate_filter = !started;
+			if (channel_count >= 3) {
+				// Center pair + Side pair
+				float xl = Vector3(-1, 0, -1).normalized().dot(rev_pos) * 0.5 + 0.5;
+				float xr = Vector3(1, 0, -1).normalized().dot(rev_pos) * 0.5 + 0.5;
 
-		if (!found) {
-			//create new if was not used before
-			if (prev_output_count < MAX_OUTPUTS) {
-				prev_outputs[prev_output_count] = prev_outputs[i]; //may be owned by another viewport
-				prev_output_count++;
+				reverb_vol.write[1].l = xl;
+				reverb_vol.write[1].r = xr;
+				reverb_vol.write[2].l = 1.0 - xr;
+				reverb_vol.write[2].r = 1.0 - xl;
 			}
-			prev_outputs[i] = current;
-			interpolate_filter = false;
-		}
-
-		//mix!
-
-		int buffers = AudioServer::get_singleton()->get_channel_count();
-
-		for (int k = 0; k < buffers; k++) {
-			AudioFrame target_volume = stream_paused_fade_out ? AudioFrame(0.f, 0.f) : current.vol[k];
-			AudioFrame vol_prev = stream_paused_fade_in ? AudioFrame(0.f, 0.f) : prev_outputs[i].vol[k];
-			AudioFrame vol_inc = (target_volume - vol_prev) / float(buffer_size);
-			AudioFrame vol = vol_prev;
 
-			if (!AudioServer::get_singleton()->thread_has_channel_mix_buffer(current.bus_index, k)) {
-				continue; //may have been deleted, will be updated on process
+			if (channel_count >= 4) {
+				// Rear pair
+				// FIXME: Not sure what math should be done here
+				float c = rev_pos.x * 0.5 + 0.5;
+				reverb_vol.write[3].l = 1.0 - c;
+				reverb_vol.write[3].r = c;
 			}
 
-			AudioFrame *target = AudioServer::get_singleton()->thread_get_channel_mix_buffer(current.bus_index, k);
-			current.filter.set_mode(AudioFilterSW::HIGHSHELF);
-			current.filter.set_sampling_rate(AudioServer::get_singleton()->get_mix_rate());
-			current.filter.set_cutoff(attenuation_filter_cutoff_hz);
-			current.filter.set_resonance(1);
-			current.filter.set_stages(1);
-			current.filter.set_gain(current.filter_gain);
-
-			if (interpolate_filter) {
-				current.filter_process[k * 2 + 0] = prev_outputs[i].filter_process[k * 2 + 0];
-				current.filter_process[k * 2 + 1] = prev_outputs[i].filter_process[k * 2 + 1];
-
-				current.filter_process[k * 2 + 0].set_filter(&current.filter, false);
-				current.filter_process[k * 2 + 1].set_filter(&current.filter, false);
-
-				current.filter_process[k * 2 + 0].update_coeffs(buffer_size);
-				current.filter_process[k * 2 + 1].update_coeffs(buffer_size);
-				for (int j = 0; j < buffer_size; j++) {
-					AudioFrame f = buffer[j] * vol;
-					current.filter_process[k * 2 + 0].process_one_interp(f.l);
-					current.filter_process[k * 2 + 1].process_one_interp(f.r);
-
-					target[j] += f;
-					vol += vol_inc;
-				}
-			} else {
-				current.filter_process[k * 2 + 0].set_filter(&current.filter);
-				current.filter_process[k * 2 + 1].set_filter(&current.filter);
-
-				current.filter_process[k * 2 + 0].update_coeffs();
-				current.filter_process[k * 2 + 1].update_coeffs();
-				for (int j = 0; j < buffer_size; j++) {
-					AudioFrame f = buffer[j] * vol;
-					current.filter_process[k * 2 + 0].process_one(f.l);
-					current.filter_process[k * 2 + 1].process_one(f.r);
-
-					target[j] += f;
-					vol += vol_inc;
-				}
+			for (int i = 0; i < channel_count; i++) {
+				reverb_vol.write[i] = reverb_vol[i].lerp(center_frame, attenuation);
 			}
-
-			if (current.reverb_bus_index >= 0) {
-				if (!AudioServer::get_singleton()->thread_has_channel_mix_buffer(current.reverb_bus_index, k)) {
-					continue; //may have been deleted, will be updated on process
-				}
-
-				AudioFrame *rtarget = AudioServer::get_singleton()->thread_get_channel_mix_buffer(current.reverb_bus_index, k);
-
-				if (current.reverb_bus_index == prev_outputs[i].reverb_bus_index) {
-					AudioFrame rvol_inc = (current.reverb_vol[k] - prev_outputs[i].reverb_vol[k]) / float(buffer_size);
-					AudioFrame rvol = prev_outputs[i].reverb_vol[k];
-
-					for (int j = 0; j < buffer_size; j++) {
-						rtarget[j] += buffer[j] * rvol;
-						rvol += rvol_inc;
-					}
-				} else {
-					AudioFrame rvol = current.reverb_vol[k];
-					for (int j = 0; j < buffer_size; j++) {
-						rtarget[j] += buffer[j] * rvol;
-					}
-				}
+		} else {
+			for (int i = 0; i < channel_count; i++) {
+				reverb_vol.write[i] = center_frame;
 			}
 		}
 
-		prev_outputs[i] = current;
-	}
-
-	prev_output_count = output_count.get();
+		for (int i = 0; i < channel_count; i++) {
+			reverb_vol.write[i] = direct_path_vol[i].lerp(reverb_vol[i] * attenuation, uniformity);
+			reverb_vol.write[i] *= area_send;
+		}
 
-	//stream is no longer active, disable this.
-	if (!stream_playback->is_playing()) {
-		active.clear();
+	} else {
+		for (int i = 0; i < 4; i++) {
+			reverb_vol.write[i] = direct_path_vol[i] * area_send;
+		}
 	}
-
-	output_ready.clear();
-	stream_paused_fade_in = false;
-	stream_paused_fade_out = false;
 }
 
 float AudioStreamPlayer3D::_get_attenuation_db(float p_distance) const {
@@ -329,14 +241,15 @@ float AudioStreamPlayer3D::_get_attenuation_db(float p_distance) const {
 void AudioStreamPlayer3D::_notification(int p_what) {
 	if (p_what == NOTIFICATION_ENTER_TREE) {
 		velocity_tracker->reset(get_global_transform().origin);
-		AudioServer::get_singleton()->add_callback(_mix_audios, this);
+		AudioServer::get_singleton()->add_listener_changed_callback(_listener_changed_cb, this);
 		if (autoplay && !Engine::get_singleton()->is_editor_hint()) {
 			play();
 		}
 	}
 
 	if (p_what == NOTIFICATION_EXIT_TREE) {
-		AudioServer::get_singleton()->remove_callback(_mix_audios, this);
+		stop();
+		AudioServer::get_singleton()->remove_listener_changed_callback(_listener_changed_cb, this);
 	}
 
 	if (p_what == NOTIFICATION_PAUSED) {
@@ -359,268 +272,216 @@ void AudioStreamPlayer3D::_notification(int p_what) {
 	if (p_what == NOTIFICATION_INTERNAL_PHYSICS_PROCESS) {
 		//update anything related to position first, if possible of course
 
-		if (!output_ready.is_set()) {
-			Vector3 linear_velocity;
+		if (!stream_playback.is_valid()) {
+			return;
+		}
+		//start playing if requested
 
-			//compute linear velocity for doppler
-			if (doppler_tracking != DOPPLER_TRACKING_DISABLED) {
-				linear_velocity = velocity_tracker->get_tracked_linear_velocity();
-			}
+		if (setplay.get() >= 0) {
+			Vector<AudioFrame> volume_vector = _update_panning();
+			AudioServer::get_singleton()->start_playback_stream(stream_playback, _get_actual_bus(), volume_vector, setplay.get());
+			active.set();
+			setplay.set(-1);
+		}
+
+		if (active.is_set() && last_mix_count != AudioServer::get_singleton()->get_mix_count()) {
+			_update_panning();
+			last_mix_count = AudioServer::get_singleton()->get_mix_count();
+		}
+
+		// Stop playing if no longer active.
+		if (!active.is_set()) {
+			set_physics_process_internal(false);
+			emit_signal(SNAME("finished"));
+		}
+	}
+}
+
+Area3D *AudioStreamPlayer3D::_get_overriding_area() {
+	//check if any area is diverting sound into a bus
+	Ref<World3D> world_3d = get_world_3d();
+	ERR_FAIL_COND_V(world_3d.is_null(), nullptr);
+
+	Vector3 global_pos = get_global_transform().origin;
 
-			Ref<World3D> world_3d = get_world_3d();
-			ERR_FAIL_COND(world_3d.is_null());
+	PhysicsDirectSpaceState3D *space_state = PhysicsServer3D::get_singleton()->space_get_direct_state(world_3d->get_space());
 
-			int new_output_count = 0;
+	PhysicsDirectSpaceState3D::ShapeResult sr[MAX_INTERSECT_AREAS];
 
-			Vector3 global_pos = get_global_transform().origin;
+	int areas = space_state->intersect_point(global_pos, sr, MAX_INTERSECT_AREAS, Set<RID>(), area_mask, false, true);
 
-			int bus_index = AudioServer::get_singleton()->thread_find_bus_index(bus);
+	for (int i = 0; i < areas; i++) {
+		if (!sr[i].collider) {
+			continue;
+		}
+
+		Area3D *tarea = Object::cast_to<Area3D>(sr[i].collider);
+		if (!tarea) {
+			continue;
+		}
+
+		if (!tarea->is_overriding_audio_bus() && !tarea->is_using_reverb_bus()) {
+			continue;
+		}
+
+		return tarea;
+	}
+	return nullptr;
+}
 
-			//check if any area is diverting sound into a bus
+StringName AudioStreamPlayer3D::_get_actual_bus() {
+	if (!stream_playback.is_valid()) {
+		return SNAME("Master");
+	}
+	Area3D *overriding_area = _get_overriding_area();
+	if (overriding_area && overriding_area->is_overriding_audio_bus() && !overriding_area->is_using_reverb_bus()) {
+		return overriding_area->get_audio_bus_name();
+	}
+	return bus;
+}
 
-			PhysicsDirectSpaceState3D *space_state = PhysicsServer3D::get_singleton()->space_get_direct_state(world_3d->get_space());
+Vector<AudioFrame> AudioStreamPlayer3D::_update_panning() {
+	Vector<AudioFrame> output_volume_vector;
+	output_volume_vector.resize(4);
+	for (AudioFrame &frame : output_volume_vector) {
+		frame = AudioFrame(0, 0);
+	}
 
-			PhysicsDirectSpaceState3D::ShapeResult sr[MAX_INTERSECT_AREAS];
+	ERR_FAIL_COND_V(stream_playback.is_null(), output_volume_vector);
 
-			int areas = space_state->intersect_point(global_pos, sr, MAX_INTERSECT_AREAS, Set<RID>(), area_mask, false, true);
-			Area3D *area = nullptr;
+	Vector3 linear_velocity;
 
-			for (int i = 0; i < areas; i++) {
-				if (!sr[i].collider) {
-					continue;
-				}
+	//compute linear velocity for doppler
+	if (doppler_tracking != DOPPLER_TRACKING_DISABLED) {
+		linear_velocity = velocity_tracker->get_tracked_linear_velocity();
+	}
 
-				Area3D *tarea = Object::cast_to<Area3D>(sr[i].collider);
-				if (!tarea) {
-					continue;
-				}
+	Vector3 global_pos = get_global_transform().origin;
 
-				if (!tarea->is_overriding_audio_bus() && !tarea->is_using_reverb_bus()) {
-					continue;
-				}
+	Ref<World3D> world_3d = get_world_3d();
+	ERR_FAIL_COND_V(world_3d.is_null(), output_volume_vector);
 
-				area = tarea;
-				break;
+	Set<Camera3D *> cameras = world_3d->get_cameras();
+	cameras.insert(get_viewport()->get_camera_3d());
+
+	PhysicsDirectSpaceState3D *space_state = PhysicsServer3D::get_singleton()->space_get_direct_state(world_3d->get_space());
+
+	for (Camera3D *camera : cameras) {
+		Viewport *vp = camera->get_viewport();
+		if (!vp->is_audio_listener_3d()) {
+			continue;
+		}
+
+		bool listener_is_camera = true;
+		Node3D *listener_node = camera;
+
+		Listener3D *listener = vp->get_listener_3d();
+		if (listener) {
+			listener_node = listener;
+			listener_is_camera = false;
+		}
+
+		Vector3 local_pos = listener_node->get_global_transform().orthonormalized().affine_inverse().xform(global_pos);
+
+		float dist = local_pos.length();
+
+		Vector3 area_sound_pos;
+		Vector3 listener_area_pos;
+
+		Area3D *area = _get_overriding_area();
+
+		if (area && area->is_using_reverb_bus() && area->get_reverb_uniformity() > 0) {
+			area_sound_pos = space_state->get_closest_point_to_object_volume(area->get_rid(), listener_node->get_global_transform().origin);
+			listener_area_pos = listener_node->get_global_transform().affine_inverse().xform(area_sound_pos);
+		}
+
+		if (max_distance > 0) {
+			float total_max = max_distance;
+
+			if (area && area->is_using_reverb_bus() && area->get_reverb_uniformity() > 0) {
+				total_max = MAX(total_max, listener_area_pos.length());
 			}
+			if (total_max > max_distance) {
+				continue; //can't hear this sound in this listener
+			}
+		}
+
+		float multiplier = Math::db2linear(_get_attenuation_db(dist));
+		if (max_distance > 0) {
+			multiplier *= MAX(0, 1.0 - (dist / max_distance));
+		}
 
-			for (const Set<Camera3D *>::Element *E = world_3d->get_cameras().front(); E; E = E->next()) {
-				Camera3D *camera = E->get();
-				Viewport *vp = camera->get_viewport();
-				if (!vp->is_audio_listener_3d()) {
-					continue;
-				}
-
-				bool listener_is_camera = true;
-				Node3D *listener_node = camera;
-
-				Listener3D *listener = vp->get_listener_3d();
-				if (listener) {
-					listener_node = listener;
-					listener_is_camera = false;
-				}
-
-				Vector3 local_pos = listener_node->get_global_transform().orthonormalized().affine_inverse().xform(global_pos);
-
-				float dist = local_pos.length();
-
-				Vector3 area_sound_pos;
-				Vector3 listener_area_pos;
-
-				if (area && area->is_using_reverb_bus() && area->get_reverb_uniformity() > 0) {
-					area_sound_pos = space_state->get_closest_point_to_object_volume(area->get_rid(), listener_node->get_global_transform().origin);
-					listener_area_pos = listener_node->to_local(area_sound_pos);
-				}
-
-				if (max_distance > 0) {
-					float total_max = max_distance;
-
-					if (area && area->is_using_reverb_bus() && area->get_reverb_uniformity() > 0) {
-						total_max = MAX(total_max, listener_area_pos.length());
-					}
-					if (total_max > max_distance) {
-						continue; //can't hear this sound in this listener
-					}
-				}
-
-				float multiplier = Math::db2linear(_get_attenuation_db(dist));
-				if (max_distance > 0) {
-					multiplier *= MAX(0, 1.0 - (dist / max_distance));
-				}
-
-				Output output;
-				output.bus_index = bus_index;
-				output.reverb_bus_index = -1; //no reverb by default
-				output.viewport = vp;
-
-				float db_att = (1.0 - MIN(1.0, multiplier)) * attenuation_filter_db;
-
-				if (emission_angle_enabled) {
-					Vector3 listenertopos = global_pos - listener_node->get_global_transform().origin;
-					float c = listenertopos.normalized().dot(get_global_transform().basis.get_axis(2).normalized()); //it's z negative
-					float angle = Math::rad2deg(Math::acos(c));
-					if (angle > emission_angle) {
-						db_att -= -emission_angle_filter_attenuation_db;
-					}
-				}
-
-				output.filter_gain = Math::db2linear(db_att);
-
-				//TODO: The lower the second parameter (tightness) the more the sound will "enclose" the listener (more undirected / playing from
-				//      speakers not facing the source) - this could be made distance dependent.
-				_calc_output_vol(local_pos.normalized(), 4.0, output);
-
-				unsigned int cc = AudioServer::get_singleton()->get_channel_count();
-				for (unsigned int k = 0; k < cc; k++) {
-					output.vol[k] *= multiplier;
-				}
-
-				bool filled_reverb = false;
-				int vol_index_max = AudioServer::get_singleton()->get_speaker_mode() + 1;
-
-				if (area) {
-					if (area->is_overriding_audio_bus()) {
-						//override audio bus
-						StringName bus_name = area->get_audio_bus_name();
-						output.bus_index = AudioServer::get_singleton()->thread_find_bus_index(bus_name);
-					}
-
-					if (area->is_using_reverb_bus()) {
-						filled_reverb = true;
-						StringName bus_name = area->get_reverb_bus();
-						output.reverb_bus_index = AudioServer::get_singleton()->thread_find_bus_index(bus_name);
-
-						float uniformity = area->get_reverb_uniformity();
-						float area_send = area->get_reverb_amount();
-
-						if (uniformity > 0.0) {
-							float distance = listener_area_pos.length();
-							float attenuation = Math::db2linear(_get_attenuation_db(distance));
-
-							//float dist_att_db = -20 * Math::log(dist + 0.00001); //logarithmic attenuation, like in real life
-
-							float center_val[3] = { 0.5f, 0.25f, 0.16666f };
-							AudioFrame center_frame(center_val[vol_index_max - 1], center_val[vol_index_max - 1]);
-
-							if (attenuation < 1.0) {
-								//pan the uniform sound
-								Vector3 rev_pos = listener_area_pos;
-								rev_pos.y = 0;
-								rev_pos.normalize();
-
-								if (cc >= 1) {
-									// Stereo pair
-									float c = rev_pos.x * 0.5 + 0.5;
-									output.reverb_vol[0].l = 1.0 - c;
-									output.reverb_vol[0].r = c;
-								}
-
-								if (cc >= 3) {
-									// Center pair + Side pair
-									float xl = Vector3(-1, 0, -1).normalized().dot(rev_pos) * 0.5 + 0.5;
-									float xr = Vector3(1, 0, -1).normalized().dot(rev_pos) * 0.5 + 0.5;
-
-									output.reverb_vol[1].l = xl;
-									output.reverb_vol[1].r = xr;
-									output.reverb_vol[2].l = 1.0 - xr;
-									output.reverb_vol[2].r = 1.0 - xl;
-								}
-
-								if (cc >= 4) {
-									// Rear pair
-									// FIXME: Not sure what math should be done here
-									float c = rev_pos.x * 0.5 + 0.5;
-									output.reverb_vol[3].l = 1.0 - c;
-									output.reverb_vol[3].r = c;
-								}
-
-								for (int i = 0; i < vol_index_max; i++) {
-									output.reverb_vol[i] = output.reverb_vol[i].lerp(center_frame, attenuation);
-								}
-							} else {
-								for (int i = 0; i < vol_index_max; i++) {
-									output.reverb_vol[i] = center_frame;
-								}
-							}
-
-							for (int i = 0; i < vol_index_max; i++) {
-								output.reverb_vol[i] = output.vol[i].lerp(output.reverb_vol[i] * attenuation, uniformity);
-								output.reverb_vol[i] *= area_send;
-							}
-
-						} else {
-							for (int i = 0; i < vol_index_max; i++) {
-								output.reverb_vol[i] = output.vol[i] * area_send;
-							}
-						}
-					}
-				}
-
-				if (doppler_tracking != DOPPLER_TRACKING_DISABLED) {
-					Vector3 listener_velocity;
-
-					if (listener_is_camera) {
-						listener_velocity = camera->get_doppler_tracked_velocity();
-					}
-
-					Vector3 local_velocity = listener_node->get_global_transform().orthonormalized().basis.xform_inv(linear_velocity - listener_velocity);
-
-					if (local_velocity == Vector3()) {
-						output.pitch_scale = 1.0;
-					} else {
-						float approaching = local_pos.normalized().dot(local_velocity.normalized());
-						float velocity = local_velocity.length();
-						float speed_of_sound = 343.0;
-
-						output.pitch_scale = speed_of_sound / (speed_of_sound + velocity * approaching);
-						output.pitch_scale = CLAMP(output.pitch_scale, (1 / 8.0), 8.0); //avoid crazy stuff
-					}
-
-				} else {
-					output.pitch_scale = 1.0;
-				}
-
-				if (!filled_reverb) {
-					for (int i = 0; i < vol_index_max; i++) {
-						output.reverb_vol[i] = AudioFrame(0, 0);
-					}
-				}
-
-				outputs[new_output_count] = output;
-				new_output_count++;
-				if (new_output_count == MAX_OUTPUTS) {
-					break;
-				}
+		float db_att = (1.0 - MIN(1.0, multiplier)) * attenuation_filter_db;
+
+		if (emission_angle_enabled) {
+			Vector3 listenertopos = global_pos - listener_node->get_global_transform().origin;
+			float c = listenertopos.normalized().dot(get_global_transform().basis.get_axis(2).normalized()); //it's z negative
+			float angle = Math::rad2deg(Math::acos(c));
+			if (angle > emission_angle) {
+				db_att -= -emission_angle_filter_attenuation_db;
 			}
+		}
+
+		AudioServer::get_singleton()->set_playback_highshelf_params(stream_playback, Math::db2linear(db_att), attenuation_filter_cutoff_hz);
+		//TODO: The lower the second parameter (tightness) the more the sound will "enclose" the listener (more undirected / playing from
+		//      speakers not facing the source) - this could be made distance dependent.
+		_calc_output_vol(local_pos.normalized(), 4.0, output_volume_vector);
 
-			output_count.set(new_output_count);
-			output_ready.set();
+		for (unsigned int k = 0; k < 4; k++) {
+			output_volume_vector.write[k] = multiplier * output_volume_vector[k];
 		}
 
-		//start playing if requested
-		if (setplay.get() >= 0.0) {
-			setseek.set(setplay.get());
-			active.set();
-			setplay.set(-1);
+		Map<StringName, Vector<AudioFrame>> bus_volumes;
+		if (area) {
+			if (area->is_overriding_audio_bus()) {
+				//override audio bus
+				bus_volumes[area->get_audio_bus_name()] = output_volume_vector;
+			}
+
+			if (area->is_using_reverb_bus()) {
+				StringName reverb_bus_name = area->get_reverb_bus();
+				Vector<AudioFrame> reverb_vol;
+				_calc_reverb_vol(area, listener_area_pos, output_volume_vector, reverb_vol);
+				bus_volumes[reverb_bus_name] = reverb_vol;
+			}
+		} else {
+			bus_volumes[bus] = output_volume_vector;
 		}
+		AudioServer::get_singleton()->set_playback_bus_volumes_linear(stream_playback, bus_volumes);
 
-		//stop playing if no longer active
-		if (!active.is_set()) {
-			set_physics_process_internal(false);
-			emit_signal(SNAME("finished"));
+		if (doppler_tracking != DOPPLER_TRACKING_DISABLED) {
+			Vector3 listener_velocity;
+
+			if (listener_is_camera) {
+				listener_velocity = camera->get_doppler_tracked_velocity();
+			}
+
+			Vector3 local_velocity = listener_node->get_global_transform().orthonormalized().basis.xform_inv(linear_velocity - listener_velocity);
+
+			if (local_velocity == Vector3()) {
+				AudioServer::get_singleton()->set_playback_pitch_scale(stream_playback, pitch_scale);
+			} else {
+				float approaching = local_pos.normalized().dot(local_velocity.normalized());
+				float velocity = local_velocity.length();
+				float speed_of_sound = 343.0;
+
+				float doppler_pitch_scale = pitch_scale * speed_of_sound / (speed_of_sound + velocity * approaching);
+				doppler_pitch_scale = CLAMP(doppler_pitch_scale, (1 / 8.0), 8.0); //avoid crazy stuff
+
+				AudioServer::get_singleton()->set_playback_pitch_scale(stream_playback, doppler_pitch_scale);
+			}
+		} else {
+			AudioServer::get_singleton()->set_playback_pitch_scale(stream_playback, pitch_scale);
 		}
 	}
+	return output_volume_vector;
 }
 
 void AudioStreamPlayer3D::set_stream(Ref<AudioStream> p_stream) {
-	AudioServer::get_singleton()->lock();
-
-	mix_buffer.resize(AudioServer::get_singleton()->thread_get_mix_buffer_size());
-
 	if (stream_playback.is_valid()) {
+		stop();
 		stream_playback.unref();
 		stream.unref();
-		active.clear();
-		setseek.set(-1);
 	}
 
 	if (p_stream.is_valid()) {
@@ -632,8 +493,6 @@ void AudioStreamPlayer3D::set_stream(Ref<AudioStream> p_stream) {
 		}
 	}
 
-	AudioServer::get_singleton()->unlock();
-
 	if (p_stream.is_valid() && stream_playback.is_null()) {
 		stream.unref();
 	}
@@ -677,27 +536,22 @@ float AudioStreamPlayer3D::get_pitch_scale() const {
 }
 
 void AudioStreamPlayer3D::play(float p_from_pos) {
-	if (!is_playing()) {
-		// Reset the prev_output_count if the stream is stopped
-		prev_output_count = 0;
-	}
-
 	if (stream_playback.is_valid()) {
 		setplay.set(p_from_pos);
-		output_ready.clear();
 		set_physics_process_internal(true);
 	}
 }
 
 void AudioStreamPlayer3D::seek(float p_seconds) {
-	if (stream_playback.is_valid()) {
-		setseek.set(p_seconds);
+	if (stream_playback.is_valid() && active.is_set()) {
+		play(p_seconds);
 	}
 }
 
 void AudioStreamPlayer3D::stop() {
 	if (stream_playback.is_valid()) {
 		active.clear();
+		AudioServer::get_singleton()->stop_playback_stream(stream_playback);
 		set_physics_process_internal(false);
 		setplay.set(-1);
 	}
@@ -713,11 +567,7 @@ bool AudioStreamPlayer3D::is_playing() const {
 
 float AudioStreamPlayer3D::get_playback_position() {
 	if (stream_playback.is_valid()) {
-		float ss = setseek.get();
-		if (ss >= 0.0) {
-			return ss;
-		}
-		return stream_playback->get_playback_position();
+		return AudioServer::get_singleton()->get_playback_position(stream_playback);
 	}
 
 	return 0;
@@ -736,7 +586,7 @@ StringName AudioStreamPlayer3D::get_bus() const {
 			return bus;
 		}
 	}
-	return "Master";
+	return SNAME("Master");
 }
 
 void AudioStreamPlayer3D::set_autoplay(bool p_enable) {
@@ -879,15 +729,16 @@ AudioStreamPlayer3D::DopplerTracking AudioStreamPlayer3D::get_doppler_tracking()
 }
 
 void AudioStreamPlayer3D::set_stream_paused(bool p_pause) {
-	if (p_pause != stream_paused) {
-		stream_paused = p_pause;
-		stream_paused_fade_in = !stream_paused;
-		stream_paused_fade_out = stream_paused;
+	if (stream_playback.is_valid()) {
+		AudioServer::get_singleton()->set_playback_paused(stream_playback, p_pause);
 	}
 }
 
 bool AudioStreamPlayer3D::get_stream_paused() const {
-	return stream_paused;
+	if (stream_playback.is_valid()) {
+		return AudioServer::get_singleton()->is_playback_paused(stream_playback);
+	}
+	return false;
 }
 
 Ref<AudioStreamPlayback> AudioStreamPlayer3D::get_stream_playback() {

scene/3d/audio_stream_player_3d.h

@@ -31,10 +31,12 @@
 #ifndef AUDIO_STREAM_PLAYER_3D_H
 #define AUDIO_STREAM_PLAYER_3D_H
 
+#include "scene/3d/area_3d.h"
 #include "scene/3d/node_3d.h"
 #include "scene/3d/velocity_tracker_3d.h"
 #include "servers/audio/audio_filter_sw.h"
 #include "servers/audio/audio_stream.h"
+#include "servers/audio_server.h"
 
 class Camera3D;
 class AudioStreamPlayer3D : public Node3D {
@@ -66,31 +68,9 @@ private:
 
 	};
 
-	struct Output {
-		AudioFilterSW filter;
-		AudioFilterSW::Processor filter_process[8];
-		AudioFrame vol[4];
-		float filter_gain = 0.0;
-		float pitch_scale = 0.0;
-		int bus_index = -1;
-		int reverb_bus_index = -1;
-		AudioFrame reverb_vol[4];
-		Viewport *viewport = nullptr; //pointer only used for reference to previous mix
-	};
-
-	Output outputs[MAX_OUTPUTS];
-	SafeNumeric<int> output_count;
-	SafeFlag output_ready;
-
-	//these are used by audio thread to have a reference of previous volumes (for ramping volume and avoiding clicks)
-	Output prev_outputs[MAX_OUTPUTS];
-	int prev_output_count = 0;
-
 	Ref<AudioStreamPlayback> stream_playback;
 	Ref<AudioStream> stream;
-	Vector<AudioFrame> mix_buffer;
 
-	SafeNumeric<float> setseek{ -1.0 };
 	SafeFlag active;
 	SafeNumeric<float> setplay{ -1.0 };
 
@@ -100,17 +80,21 @@ private:
 	float max_db = 3.0;
 	float pitch_scale = 1.0;
 	bool autoplay = false;
-	bool stream_paused = false;
-	bool stream_paused_fade_in = false;
-	bool stream_paused_fade_out = false;
-	StringName bus;
+	StringName bus = "Master";
+
+	uint64_t last_mix_count = -1;
+
+	static void _calc_output_vol(const Vector3 &source_dir, real_t tightness, Vector<AudioFrame> &output);
+
+	void _calc_reverb_vol(Area3D *area, Vector3 listener_area_pos, Vector<AudioFrame> direct_path_vol, Vector<AudioFrame> &reverb_vol);
 
-	static void _calc_output_vol(const Vector3 &source_dir, real_t tightness, Output &output);
-	void _mix_audio();
-	static void _mix_audios(void *self) { reinterpret_cast<AudioStreamPlayer3D *>(self)->_mix_audio(); }
+	static void _listener_changed_cb(void *self) { reinterpret_cast<AudioStreamPlayer3D *>(self)->_update_panning(); }
 
 	void _set_playing(bool p_enable);
 	bool _is_active() const;
+	StringName _get_actual_bus();
+	Area3D *_get_overriding_area();
+	Vector<AudioFrame> _update_panning();
 
 	void _bus_layout_changed();
 

scene/audio/audio_stream_player.cpp

@@ -31,119 +31,18 @@
 #include "audio_stream_player.h"
 
 #include "core/config/engine.h"
-
-void AudioStreamPlayer::_mix_to_bus(const AudioFrame *p_frames, int p_amount) {
-	int bus_index = AudioServer::get_singleton()->thread_find_bus_index(bus);
-
-	AudioFrame *targets[4] = { nullptr, nullptr, nullptr, nullptr };
-
-	if (AudioServer::get_singleton()->get_speaker_mode() == AudioServer::SPEAKER_MODE_STEREO) {
-		targets[0] = AudioServer::get_singleton()->thread_get_channel_mix_buffer(bus_index, 0);
-	} else {
-		switch (mix_target) {
-			case MIX_TARGET_STEREO: {
-				targets[0] = AudioServer::get_singleton()->thread_get_channel_mix_buffer(bus_index, 0);
-			} break;
-			case MIX_TARGET_SURROUND: {
-				for (int i = 0; i < AudioServer::get_singleton()->get_channel_count(); i++) {
-					targets[i] = AudioServer::get_singleton()->thread_get_channel_mix_buffer(bus_index, i);
-				}
-			} break;
-			case MIX_TARGET_CENTER: {
-				targets[0] = AudioServer::get_singleton()->thread_get_channel_mix_buffer(bus_index, 1);
-			} break;
-		}
-	}
-
-	for (int c = 0; c < 4; c++) {
-		if (!targets[c]) {
-			break;
-		}
-		for (int i = 0; i < p_amount; i++) {
-			targets[c][i] += p_frames[i];
-		}
-	}
-}
-
-void AudioStreamPlayer::_mix_internal(bool p_fadeout) {
-	//get data
-	AudioFrame *buffer = mix_buffer.ptrw();
-	int buffer_size = mix_buffer.size();
-
-	if (p_fadeout) {
-		// Short fadeout ramp
-		buffer_size = MIN(buffer_size, 128);
-	}
-
-	stream_playback->mix(buffer, pitch_scale, buffer_size);
-
-	//multiply volume interpolating to avoid clicks if this changes
-	float target_volume = p_fadeout ? -80.0 : volume_db;
-	float vol = Math::db2linear(mix_volume_db);
-	float vol_inc = (Math::db2linear(target_volume) - vol) / float(buffer_size);
-
-	for (int i = 0; i < buffer_size; i++) {
-		buffer[i] *= vol;
-		vol += vol_inc;
-	}
-
-	//set volume for next mix
-	mix_volume_db = target_volume;
-
-	_mix_to_bus(buffer, buffer_size);
-}
-
-void AudioStreamPlayer::_mix_audio() {
-	if (use_fadeout) {
-		_mix_to_bus(fadeout_buffer.ptr(), fadeout_buffer.size());
-		use_fadeout = false;
-	}
-
-	if (!stream_playback.is_valid() || !active.is_set() ||
-			(stream_paused && !stream_paused_fade)) {
-		return;
-	}
-
-	if (stream_paused) {
-		if (stream_paused_fade && stream_playback->is_playing()) {
-			_mix_internal(true);
-			stream_paused_fade = false;
-		}
-		return;
-	}
-
-	if (setstop.is_set()) {
-		_mix_internal(true);
-		stream_playback->stop();
-		setstop.clear();
-	}
-
-	if (setseek.get() >= 0.0 && !stop_has_priority.is_set()) {
-		if (stream_playback->is_playing()) {
-			//fade out to avoid pops
-			_mix_internal(true);
-		}
-
-		stream_playback->start(setseek.get());
-		setseek.set(-1.0); //reset seek
-		mix_volume_db = volume_db; //reset ramp
-	}
-
-	stop_has_priority.clear();
-
-	_mix_internal(false);
-}
+#include "core/math/audio_frame.h"
+#include "servers/audio_server.h"
 
 void AudioStreamPlayer::_notification(int p_what) {
 	if (p_what == NOTIFICATION_ENTER_TREE) {
-		AudioServer::get_singleton()->add_callback(_mix_audios, this);
 		if (autoplay && !Engine::get_singleton()->is_editor_hint()) {
 			play();
 		}
 	}
 
 	if (p_what == NOTIFICATION_INTERNAL_PROCESS) {
-		if (!active.is_set() || (setseek.get() < 0 && !stream_playback->is_playing())) {
+		if (stream_playback.is_valid() && active.is_set() && !AudioServer::get_singleton()->is_playback_active(stream_playback)) {
 			active.clear();
 			set_process_internal(false);
 			emit_signal(SNAME("finished"));
@@ -151,7 +50,9 @@ void AudioStreamPlayer::_notification(int p_what) {
 	}
 
 	if (p_what == NOTIFICATION_EXIT_TREE) {
-		AudioServer::get_singleton()->remove_callback(_mix_audios, this);
+		if (stream_playback.is_valid()) {
+			AudioServer::get_singleton()->stop_playback_stream(stream_playback);
+		}
 	}
 
 	if (p_what == NOTIFICATION_PAUSED) {
@@ -167,38 +68,10 @@ void AudioStreamPlayer::_notification(int p_what) {
 }
 
 void AudioStreamPlayer::set_stream(Ref<AudioStream> p_stream) {
-	AudioServer::get_singleton()->lock();
-
-	if (active.is_set() && stream_playback.is_valid() && !stream_paused) {
-		//changing streams out of the blue is not a great idea, but at least
-		//let's try to somehow avoid a click
-
-		AudioFrame *buffer = fadeout_buffer.ptrw();
-		int buffer_size = fadeout_buffer.size();
-
-		stream_playback->mix(buffer, pitch_scale, buffer_size);
-
-		//multiply volume interpolating to avoid clicks if this changes
-		float target_volume = -80.0;
-		float vol = Math::db2linear(mix_volume_db);
-		float vol_inc = (Math::db2linear(target_volume) - vol) / float(buffer_size);
-
-		for (int i = 0; i < buffer_size; i++) {
-			buffer[i] *= vol;
-			vol += vol_inc;
-		}
-
-		use_fadeout = true;
-	}
-
-	mix_buffer.resize(AudioServer::get_singleton()->thread_get_mix_buffer_size());
-
 	if (stream_playback.is_valid()) {
+		stop();
 		stream_playback.unref();
 		stream.unref();
-		active.clear();
-		setseek.set(-1);
-		setstop.clear();
 	}
 
 	if (p_stream.is_valid()) {
@@ -210,8 +83,6 @@ void AudioStreamPlayer::set_stream(Ref<AudioStream> p_stream) {
 		}
 	}
 
-	AudioServer::get_singleton()->unlock();
-
 	if (p_stream.is_valid() && stream_playback.is_null()) {
 		stream.unref();
 	}
@@ -223,6 +94,8 @@ Ref<AudioStream> AudioStreamPlayer::get_stream() const {
 
 void AudioStreamPlayer::set_volume_db(float p_volume) {
 	volume_db = p_volume;
+
+	AudioServer::get_singleton()->set_playback_all_bus_volumes_linear(stream_playback, _get_volume_vector());
 }
 
 float AudioStreamPlayer::get_volume_db() const {
@@ -232,6 +105,8 @@ float AudioStreamPlayer::get_volume_db() const {
 void AudioStreamPlayer::set_pitch_scale(float p_pitch_scale) {
 	ERR_FAIL_COND(p_pitch_scale <= 0.0);
 	pitch_scale = p_pitch_scale;
+
+	AudioServer::get_singleton()->set_playback_pitch_scale(stream_playback, pitch_scale);
 }
 
 float AudioStreamPlayer::get_pitch_scale() const {
@@ -239,31 +114,32 @@ float AudioStreamPlayer::get_pitch_scale() const {
 }
 
 void AudioStreamPlayer::play(float p_from_pos) {
+	stop();
+	if (stream.is_valid()) {
+		stream_playback = stream->instance_playback();
+	}
 	if (stream_playback.is_valid()) {
-		//mix_volume_db = volume_db; do not reset volume ramp here, can cause clicks
-		setseek.set(p_from_pos);
-		stop_has_priority.clear();
+		AudioServer::get_singleton()->start_playback_stream(stream_playback, bus, _get_volume_vector(), p_from_pos);
 		active.set();
-		set_process_internal(true);
 	}
 }
 
 void AudioStreamPlayer::seek(float p_seconds) {
-	if (stream_playback.is_valid()) {
-		setseek.set(p_seconds);
+	if (stream_playback.is_valid() && active.is_set()) {
+		play(p_seconds);
 	}
 }
 
 void AudioStreamPlayer::stop() {
-	if (stream_playback.is_valid() && active.is_set()) {
-		setstop.set();
-		stop_has_priority.set();
+	if (stream_playback.is_valid()) {
+		active.clear();
+		AudioServer::get_singleton()->stop_playback_stream(stream_playback);
 	}
 }
 
 bool AudioStreamPlayer::is_playing() const {
 	if (stream_playback.is_valid()) {
-		return active.is_set() && !setstop.is_set(); //&& stream_playback->is_playing();
+		return AudioServer::get_singleton()->is_playback_active(stream_playback);
 	}
 
 	return false;
@@ -271,26 +147,22 @@ bool AudioStreamPlayer::is_playing() const {
 
 float AudioStreamPlayer::get_playback_position() {
 	if (stream_playback.is_valid()) {
-		float ss = setseek.get();
-		if (ss >= 0.0) {
-			return ss;
-		}
-		return stream_playback->get_playback_position();
+		return AudioServer::get_singleton()->get_playback_position(stream_playback);
 	}
 
 	return 0;
 }
 
 void AudioStreamPlayer::set_bus(const StringName &p_bus) {
-	//if audio is active, must lock this
-	AudioServer::get_singleton()->lock();
 	bus = p_bus;
-	AudioServer::get_singleton()->unlock();
+	if (stream_playback.is_valid()) {
+		AudioServer::get_singleton()->set_playback_bus_exclusive(stream_playback, p_bus, _get_volume_vector());
+	}
 }
 
 StringName AudioStreamPlayer::get_bus() const {
 	for (int i = 0; i < AudioServer::get_singleton()->get_bus_count(); i++) {
-		if (AudioServer::get_singleton()->get_bus_name(i) == bus) {
+		if (AudioServer::get_singleton()->get_bus_name(i) == String(bus)) {
 			return bus;
 		}
 	}
@@ -322,18 +194,61 @@ void AudioStreamPlayer::_set_playing(bool p_enable) {
 }
 
 bool AudioStreamPlayer::_is_active() const {
-	return active.is_set();
+	if (stream_playback.is_valid()) {
+		return AudioServer::get_singleton()->is_playback_active(stream_playback);
+	}
+	return false;
 }
 
 void AudioStreamPlayer::set_stream_paused(bool p_pause) {
-	if (p_pause != stream_paused) {
-		stream_paused = p_pause;
-		stream_paused_fade = p_pause;
+	// TODO this does not have perfect recall, fix that maybe? If the stream isn't set, we can't persist this bool.
+	if (stream_playback.is_valid()) {
+		AudioServer::get_singleton()->set_playback_paused(stream_playback, p_pause);
 	}
 }
 
 bool AudioStreamPlayer::get_stream_paused() const {
-	return stream_paused;
+	if (stream_playback.is_valid()) {
+		return AudioServer::get_singleton()->is_playback_paused(stream_playback);
+	}
+	return false;
+}
+
+Vector<AudioFrame> AudioStreamPlayer::_get_volume_vector() {
+	Vector<AudioFrame> volume_vector;
+	// We need at most four stereo pairs (for 7.1 systems).
+	volume_vector.resize(4);
+
+	// Initialize the volume vector to zero.
+	for (AudioFrame &channel_volume_db : volume_vector) {
+		channel_volume_db = AudioFrame(0, 0);
+	}
+
+	float volume_linear = Math::db2linear(volume_db);
+
+	// Set the volume vector up according to the speaker mode and mix target.
+	// TODO do we need to scale the volume down when we output to more channels?
+	if (AudioServer::get_singleton()->get_speaker_mode() == AudioServer::SPEAKER_MODE_STEREO) {
+		volume_vector.write[0] = AudioFrame(volume_linear, volume_linear);
+	} else {
+		switch (mix_target) {
+			case MIX_TARGET_STEREO: {
+				volume_vector.write[0] = AudioFrame(volume_linear, volume_linear);
+			} break;
+			case MIX_TARGET_SURROUND: {
+				// TODO Make sure this is right.
+				volume_vector.write[0] = AudioFrame(volume_linear, volume_linear);
+				volume_vector.write[1] = AudioFrame(volume_linear, /* LFE= */ 1.0f);
+				volume_vector.write[2] = AudioFrame(volume_linear, volume_linear);
+				volume_vector.write[3] = AudioFrame(volume_linear, volume_linear);
+			} break;
+			case MIX_TARGET_CENTER: {
+				// TODO Make sure this is right.
+				volume_vector.write[1] = AudioFrame(volume_linear, /* LFE= */ 1.0f);
+			} break;
+		}
+	}
+	return volume_vector;
 }
 
 void AudioStreamPlayer::_validate_property(PropertyInfo &property) const {
@@ -410,8 +325,6 @@ void AudioStreamPlayer::_bind_methods() {
 }
 
 AudioStreamPlayer::AudioStreamPlayer() {
-	fadeout_buffer.resize(512);
-
 	AudioServer::get_singleton()->connect("bus_layout_changed", callable_mp(this, &AudioStreamPlayer::_bus_layout_changed));
 }
 

scene/audio/audio_stream_player.h

@@ -48,22 +48,13 @@ public:
 private:
 	Ref<AudioStreamPlayback> stream_playback;
 	Ref<AudioStream> stream;
-	Vector<AudioFrame> mix_buffer;
-	Vector<AudioFrame> fadeout_buffer;
-	bool use_fadeout = false;
 
-	SafeNumeric<float> setseek{ -1.0 };
 	SafeFlag active;
-	SafeFlag setstop;
-	SafeFlag stop_has_priority;
 
-	float mix_volume_db = 0.0;
 	float pitch_scale = 1.0;
 	float volume_db = 0.0;
 	bool autoplay = false;
-	bool stream_paused = false;
-	bool stream_paused_fade = false;
-	StringName bus;
+	StringName bus = "Master";
 
 	MixTarget mix_target = MIX_TARGET_STEREO;
 
@@ -77,6 +68,8 @@ private:
 	void _bus_layout_changed();
 	void _mix_to_bus(const AudioFrame *p_frames, int p_amount);
 
+	Vector<AudioFrame> _get_volume_vector();
+
 protected:
 	void _validate_property(PropertyInfo &property) const override;
 	void _notification(int p_what);

scene/gui/video_player.cpp

@@ -129,7 +129,7 @@ void VideoPlayer::_mix_audio() {
 void VideoPlayer::_notification(int p_notification) {
 	switch (p_notification) {
 		case NOTIFICATION_ENTER_TREE: {
-			AudioServer::get_singleton()->add_callback(_mix_audios, this);
+			AudioServer::get_singleton()->add_mix_callback(_mix_audios, this);
 
 			if (stream.is_valid() && autoplay && !Engine::get_singleton()->is_editor_hint()) {
 				play();
@@ -138,8 +138,7 @@ void VideoPlayer::_notification(int p_notification) {
 		} break;
 
 		case NOTIFICATION_EXIT_TREE: {
-			AudioServer::get_singleton()->remove_callback(_mix_audios, this);
-
+			AudioServer::get_singleton()->remove_mix_callback(_mix_audios, this);
 		} break;
 
 		case NOTIFICATION_INTERNAL_PROCESS: {

scene/main/viewport.cpp

@@ -52,6 +52,7 @@
 #include "scene/resources/text_line.h"
 #include "scene/resources/world_2d.h"
 #include "scene/scene_string_names.h"
+#include "servers/audio_server.h"
 
 void ViewportTexture::setup_local_to_scene() {
 	if (vp) {
@@ -820,12 +821,7 @@ Rect2 Viewport::get_visible_rect() const {
 }
 
 void Viewport::_update_listener_2d() {
-	/*
-	if (is_inside_tree() && audio_listener_3d && (!get_parent() || (Object::cast_to<Control>(get_parent()) && Object::cast_to<Control>(get_parent())->is_visible_in_tree())))
-		SpatialSound2DServer::get_singleton()->listener_set_space(internal_listener_2d, find_world_2d()->get_sound_space());
-	else
-		SpatialSound2DServer::get_singleton()->listener_set_space(internal_listener_2d, RID());
-*/
+	AudioServer::get_singleton()->notify_listener_changed();
 }
 
 void Viewport::set_as_audio_listener_2d(bool p_enable) {
@@ -3072,6 +3068,7 @@ bool Viewport::is_audio_listener_3d() const {
 }
 
 void Viewport::_update_listener_3d() {
+	AudioServer::get_singleton()->notify_listener_changed();
 }
 
 void Viewport::_listener_transform_3d_changed_notify() {

servers/audio_server.cpp

@@ -32,13 +32,19 @@
 
 #include "core/config/project_settings.h"
 #include "core/debugger/engine_debugger.h"
+#include "core/error/error_macros.h"
 #include "core/io/file_access.h"
 #include "core/io/resource_loader.h"
+#include "core/math/audio_frame.h"
 #include "core/os/os.h"
+#include "core/string/string_name.h"
+#include "core/templates/pair.h"
 #include "scene/resources/audio_stream_sample.h"
 #include "servers/audio/audio_driver_dummy.h"
 #include "servers/audio/effects/audio_effect_compressor.h"
 
+#include <cstring>
+
 #ifdef TOOLS_ENABLED
 #define MARK_EDITED set_edited(true);
 #else
@@ -234,6 +240,7 @@ AudioDriver *AudioDriverManager::get_driver(int p_driver) {
 //////////////////////////////////////////////
 
 void AudioServer::_driver_process(int p_frames, int32_t *p_buffer) {
+	mix_count++;
 	int todo = p_frames;
 
 #ifdef DEBUG_ENABLED
@@ -331,10 +338,156 @@ void AudioServer::_mix_step() {
 			bus->soloed = false;
 		}
 	}
+	for (CallbackItem *ci : mix_callback_list) {
+		ci->callback(ci->userdata);
+	}
+
+	for (AudioStreamPlaybackListNode *playback : playback_list) {
+		// Paused streams are no-ops. Don't even mix audio from the stream playback.
+		if (playback->state.load() == AudioStreamPlaybackListNode::PAUSED) {
+			continue;
+		}
+
+		bool fading_out = playback->state.load() == AudioStreamPlaybackListNode::FADE_OUT_TO_DELETION || playback->state.load() == AudioStreamPlaybackListNode::FADE_OUT_TO_PAUSE;
+
+		AudioFrame *buf = mix_buffer.ptrw();
+
+		// Copy the lookeahead buffer into the mix buffer.
+		for (int i = 0; i < LOOKAHEAD_BUFFER_SIZE; i++) {
+			buf[i] = playback->lookahead[i];
+		}
+
+		// Mix the audio stream
+		unsigned int mixed_frames = playback->stream_playback->mix(&buf[LOOKAHEAD_BUFFER_SIZE], playback->pitch_scale.get(), buffer_size);
+
+		if (mixed_frames != buffer_size) {
+			// We know we have at least the size of our lookahead buffer for fade-out purposes.
+
+			float fadeout_base = 0.87;
+			float fadeout_coefficient = 1;
+			static_assert(LOOKAHEAD_BUFFER_SIZE == 32, "Update fadeout_base and comment here if you change LOOKAHEAD_BUFFER_SIZE.");
+			// 0.87 ^ 32 = 0.0116. There might still be a pop but it'll be way better than if we didn't do this.
+			for (unsigned int idx = mixed_frames; idx < buffer_size; idx++) {
+				fadeout_coefficient *= fadeout_base;
+				buf[idx] *= fadeout_coefficient;
+			}
+			AudioStreamPlaybackListNode::PlaybackState new_state;
+			new_state = AudioStreamPlaybackListNode::AWAITING_DELETION;
+			playback->state.store(new_state);
+		} else {
+			// Move the last little bit of what we just mixed into our lookahead buffer.
+			for (int i = 0; i < LOOKAHEAD_BUFFER_SIZE; i++) {
+				playback->lookahead[i] = buf[buffer_size + i];
+			}
+		}
+
+		ERR_FAIL_COND(playback->bus_details.load() == nullptr);
+		// By putting null into the bus details pointers, we're taking ownership of their memory for the duration of this mix.
+		AudioStreamPlaybackBusDetails *bus_details = nullptr;
+		{
+			std::atomic<AudioStreamPlaybackBusDetails *> bus_details_atomic = nullptr;
+			bus_details = playback->bus_details.exchange(bus_details_atomic);
+		}
+		ERR_FAIL_COND(bus_details == nullptr);
+		AudioStreamPlaybackBusDetails *prev_bus_details = playback->prev_bus_details;
+
+		// Mix to any active buses.
+		for (int idx = 0; idx < MAX_BUSES_PER_PLAYBACK; idx++) {
+			if (!bus_details->bus_active[idx]) {
+				continue;
+			}
+			int bus_idx = thread_find_bus_index(bus_details->bus[idx]);
+
+			int prev_bus_idx = -1;
+			for (int search_idx = 0; search_idx < MAX_BUSES_PER_PLAYBACK; search_idx++) {
+				if (!prev_bus_details->bus_active[search_idx]) {
+					continue;
+				}
+				if (prev_bus_details->bus[search_idx].hash() == bus_details->bus[idx].hash()) {
+					prev_bus_idx = search_idx;
+				}
+			}
+
+			for (int channel_idx = 0; channel_idx < channel_count; channel_idx++) {
+				AudioFrame *channel_buf = thread_get_channel_mix_buffer(bus_idx, channel_idx);
+				if (fading_out) {
+					bus_details->volume[idx][channel_idx] = AudioFrame(0, 0);
+				}
+				AudioFrame channel_vol = bus_details->volume[idx][channel_idx];
+
+				AudioFrame prev_channel_vol = AudioFrame(0, 0);
+				if (prev_bus_idx != -1) {
+					prev_channel_vol = prev_bus_details->volume[prev_bus_idx][channel_idx];
+				}
+				_mix_step_for_channel(channel_buf, buf, prev_channel_vol, channel_vol, playback->attenuation_filter_cutoff_hz.get(), playback->highshelf_gain.get(), &playback->filter_process[channel_idx * 2], &playback->filter_process[channel_idx * 2 + 1]);
+			}
+		}
+
+		// Now go through and fade-out any buses that were being played to previously that we missed by going through current data.
+		for (int idx = 0; idx < MAX_BUSES_PER_PLAYBACK; idx++) {
+			if (!prev_bus_details->bus_active[idx]) {
+				continue;
+			}
+			int bus_idx = thread_find_bus_index(prev_bus_details->bus[idx]);
+
+			int current_bus_idx = -1;
+			for (int search_idx = 0; search_idx < MAX_BUSES_PER_PLAYBACK; search_idx++) {
+				if (bus_details->bus[search_idx] == prev_bus_details->bus[idx]) {
+					current_bus_idx = search_idx;
+				}
+			}
+			if (current_bus_idx != -1) {
+				// If we found a corresponding bus in the current bus assignments, we've already mixed to this bus.
+				continue;
+			}
+
+			for (int channel_idx = 0; channel_idx < channel_count; channel_idx++) {
+				AudioFrame *channel_buf = thread_get_channel_mix_buffer(bus_idx, channel_idx);
+				AudioFrame prev_channel_vol = prev_bus_details->volume[idx][channel_idx];
+				// Fade out to silence
+				_mix_step_for_channel(channel_buf, buf, prev_channel_vol, AudioFrame(0, 0), playback->attenuation_filter_cutoff_hz.get(), playback->highshelf_gain.get(), &playback->filter_process[channel_idx * 2], &playback->filter_process[channel_idx * 2 + 1]);
+			}
+		}
+
+		// Copy the bus details we mixed with to the previous bus details to maintain volume ramps.
+		std::copy(std::begin(bus_details->bus_active), std::end(bus_details->bus_active), std::begin(prev_bus_details->bus_active));
+		std::copy(std::begin(bus_details->bus), std::end(bus_details->bus), std::begin(prev_bus_details->bus));
+		for (int bus_idx = 0; bus_idx < MAX_BUSES_PER_PLAYBACK; bus_idx++) {
+			std::copy(std::begin(bus_details->volume[bus_idx]), std::end(bus_details->volume[bus_idx]), std::begin(prev_bus_details->volume[bus_idx]));
+		}
+
+		AudioStreamPlaybackBusDetails *bus_details_expected = nullptr;
+		// Only put the bus details pointer back if it hasn't been updated already.
+		if (!playback->bus_details.compare_exchange_strong(/* expected= */ bus_details_expected, /* new= */ bus_details)) {
+			// If it *has* been updated already, queue the old one for deletion.
+			bus_details_graveyard.insert(bus_details);
+		}
 
-	//make callbacks for mixing the audio
-	for (Set<CallbackItem>::Element *E = callbacks.front(); E; E = E->next()) {
-		E->get().callback(E->get().userdata);
+		switch (playback->state.load()) {
+			case AudioStreamPlaybackListNode::AWAITING_DELETION:
+			case AudioStreamPlaybackListNode::FADE_OUT_TO_DELETION:
+				playback_list.erase(playback, [](AudioStreamPlaybackListNode *p) {
+					if (p->prev_bus_details)
+						delete p->prev_bus_details;
+					if (p->bus_details)
+						delete p->bus_details;
+					p->stream_playback.unref();
+					delete p;
+				});
+				break;
+			case AudioStreamPlaybackListNode::FADE_OUT_TO_PAUSE: {
+				// Pause the stream.
+				AudioStreamPlaybackListNode::PlaybackState old_state, new_state;
+				do {
+					old_state = playback->state.load();
+					new_state = AudioStreamPlaybackListNode::PAUSED;
+				} while (!playback->state.compare_exchange_strong(/* expected= */ old_state, new_state));
+			} break;
+			case AudioStreamPlaybackListNode::PLAYING:
+			case AudioStreamPlaybackListNode::PAUSED:
+				// No-op!
+				break;
+		}
 	}
 
 	for (int i = buses.size() - 1; i >= 0; i--) {
@@ -464,6 +617,53 @@ void AudioServer::_mix_step() {
 	to_mix = buffer_size;
 }
 
+void AudioServer::_mix_step_for_channel(AudioFrame *p_out_buf, AudioFrame *p_source_buf, AudioFrame p_vol_start, AudioFrame p_vol_final, float p_attenuation_filter_cutoff_hz, float p_highshelf_gain, AudioFilterSW::Processor *p_processor_l, AudioFilterSW::Processor *p_processor_r) {
+	if (p_highshelf_gain != 0) {
+		AudioFilterSW filter;
+		filter.set_mode(AudioFilterSW::HIGHSHELF);
+		filter.set_sampling_rate(AudioServer::get_singleton()->get_mix_rate());
+		filter.set_cutoff(p_attenuation_filter_cutoff_hz);
+		filter.set_resonance(1);
+		filter.set_stages(1);
+		filter.set_gain(p_highshelf_gain);
+
+		ERR_FAIL_COND(p_processor_l == nullptr);
+		ERR_FAIL_COND(p_processor_r == nullptr);
+
+		bool is_just_started = p_vol_start.l == 0 && p_vol_start.r == 0;
+		p_processor_l->set_filter(&filter, /* clear_history= */ is_just_started);
+		p_processor_l->update_coeffs(buffer_size);
+		p_processor_r->set_filter(&filter, /* clear_history= */ is_just_started);
+		p_processor_r->update_coeffs(buffer_size);
+
+		for (unsigned int frame_idx = 0; frame_idx < buffer_size; frame_idx++) {
+			// Make this buffer size invariant if buffer_size ever becomes a project setting.
+			float lerp_param = (float)frame_idx / buffer_size;
+			AudioFrame vol = p_vol_final * lerp_param + (1 - lerp_param) * p_vol_start;
+			AudioFrame mixed = vol * p_source_buf[frame_idx];
+			p_processor_l->process_one_interp(mixed.l);
+			p_processor_r->process_one_interp(mixed.r);
+			p_out_buf[frame_idx] += mixed;
+		}
+
+	} else {
+		for (unsigned int frame_idx = 0; frame_idx < buffer_size; frame_idx++) {
+			// Make this buffer size invariant if buffer_size ever becomes a project setting.
+			float lerp_param = (float)frame_idx / buffer_size;
+			p_out_buf[frame_idx] += (p_vol_final * lerp_param + (1 - lerp_param) * p_vol_start) * p_source_buf[frame_idx];
+		}
+	}
+}
+
+AudioServer::AudioStreamPlaybackListNode *AudioServer::_find_playback_list_node(Ref<AudioStreamPlayback> p_playback) {
+	for (AudioStreamPlaybackListNode *playback_list_node : playback_list) {
+		if (playback_list_node->stream_playback == p_playback) {
+			return playback_list_node;
+		}
+	}
+	return nullptr;
+}
+
 bool AudioServer::thread_has_channel_mix_buffer(int p_bus, int p_buffer) const {
 	if (p_bus < 0 || p_bus >= buses.size()) {
 		return false;
@@ -923,9 +1123,216 @@ float AudioServer::get_playback_speed_scale() const {
 	return playback_speed_scale;
 }
 
+void AudioServer::start_playback_stream(Ref<AudioStreamPlayback> p_playback, StringName p_bus, Vector<AudioFrame> p_volume_db_vector, float p_start_time) {
+	ERR_FAIL_COND(p_playback.is_null());
+
+	Map<StringName, Vector<AudioFrame>> map;
+	map[p_bus] = p_volume_db_vector;
+
+	start_playback_stream(p_playback, map, p_start_time);
+}
+
+void AudioServer::start_playback_stream(Ref<AudioStreamPlayback> p_playback, Map<StringName, Vector<AudioFrame>> p_bus_volumes, float p_start_time) {
+	ERR_FAIL_COND(p_playback.is_null());
+
+	AudioStreamPlaybackListNode *playback_node = new AudioStreamPlaybackListNode();
+	playback_node->stream_playback = p_playback;
+	playback_node->stream_playback->start(p_start_time);
+
+	AudioStreamPlaybackBusDetails *new_bus_details = new AudioStreamPlaybackBusDetails();
+	int idx = 0;
+	for (KeyValue<StringName, Vector<AudioFrame>> pair : p_bus_volumes) {
+		ERR_FAIL_COND(pair.value.size() < channel_count);
+		ERR_FAIL_COND(pair.value.size() != MAX_CHANNELS_PER_BUS);
+
+		new_bus_details->bus_active[idx] = true;
+		new_bus_details->bus[idx] = pair.key;
+		for (int channel_idx = 0; channel_idx < MAX_CHANNELS_PER_BUS; channel_idx++) {
+			new_bus_details->volume[idx][channel_idx] = pair.value[channel_idx];
+		}
+	}
+	playback_node->bus_details = new_bus_details;
+	playback_node->prev_bus_details = new AudioStreamPlaybackBusDetails();
+
+	playback_node->setseek.set(-1);
+	playback_node->pitch_scale.set(1);
+	playback_node->highshelf_gain.set(0);
+	playback_node->attenuation_filter_cutoff_hz.set(0);
+
+	memset(playback_node->prev_bus_details->volume, 0, sizeof(playback_node->prev_bus_details->volume));
+
+	for (AudioFrame &frame : playback_node->lookahead) {
+		frame = AudioFrame(0, 0);
+	}
+
+	playback_node->state.store(AudioStreamPlaybackListNode::PLAYING);
+
+	playback_list.insert(playback_node);
+}
+
+void AudioServer::stop_playback_stream(Ref<AudioStreamPlayback> p_playback) {
+	ERR_FAIL_COND(p_playback.is_null());
+
+	AudioStreamPlaybackListNode *playback_node = _find_playback_list_node(p_playback);
+	if (!playback_node) {
+		return;
+	}
+
+	AudioStreamPlaybackListNode::PlaybackState new_state, old_state;
+	do {
+		old_state = playback_node->state.load();
+		new_state = AudioStreamPlaybackListNode::FADE_OUT_TO_DELETION;
+
+	} while (!playback_node->state.compare_exchange_strong(old_state, new_state));
+}
+
+void AudioServer::set_playback_bus_exclusive(Ref<AudioStreamPlayback> p_playback, StringName p_bus, Vector<AudioFrame> p_volumes) {
+	ERR_FAIL_COND(p_volumes.size() != MAX_CHANNELS_PER_BUS);
+
+	Map<StringName, Vector<AudioFrame>> map;
+	map[p_bus] = p_volumes;
+
+	set_playback_bus_volumes_linear(p_playback, map);
+}
+
+void AudioServer::set_playback_bus_volumes_linear(Ref<AudioStreamPlayback> p_playback, Map<StringName, Vector<AudioFrame>> p_bus_volumes) {
+	ERR_FAIL_COND(p_bus_volumes.size() > MAX_BUSES_PER_PLAYBACK);
+
+	AudioStreamPlaybackListNode *playback_node = _find_playback_list_node(p_playback);
+	if (!playback_node) {
+		return;
+	}
+	AudioStreamPlaybackBusDetails *old_bus_details, *new_bus_details = new AudioStreamPlaybackBusDetails();
+
+	int idx = 0;
+	for (KeyValue<StringName, Vector<AudioFrame>> pair : p_bus_volumes) {
+		ERR_FAIL_COND(pair.value.size() < channel_count);
+		ERR_FAIL_COND(pair.value.size() != MAX_CHANNELS_PER_BUS);
+
+		new_bus_details->bus_active[idx] = true;
+		new_bus_details->bus[idx] = pair.key;
+		for (int channel_idx = 0; channel_idx < MAX_CHANNELS_PER_BUS; channel_idx++) {
+			new_bus_details->volume[idx][channel_idx] = pair.value[channel_idx];
+		}
+	}
+
+	do {
+		old_bus_details = playback_node->bus_details.load();
+	} while (!playback_node->bus_details.compare_exchange_strong(old_bus_details, new_bus_details));
+
+	bus_details_graveyard.insert(old_bus_details);
+}
+
+void AudioServer::set_playback_all_bus_volumes_linear(Ref<AudioStreamPlayback> p_playback, Vector<AudioFrame> p_volumes) {
+	ERR_FAIL_COND(p_playback.is_null());
+	ERR_FAIL_COND(p_volumes.size() != MAX_CHANNELS_PER_BUS);
+
+	Map<StringName, Vector<AudioFrame>> map;
+
+	AudioStreamPlaybackListNode *playback_node = _find_playback_list_node(p_playback);
+	if (!playback_node) {
+		return;
+	}
+	for (int bus_idx = 0; bus_idx < MAX_BUSES_PER_PLAYBACK; bus_idx++) {
+		if (playback_node->bus_details.load()->bus_active[bus_idx]) {
+			map[playback_node->bus_details.load()->bus[bus_idx]] = p_volumes;
+		}
+	}
+
+	set_playback_bus_volumes_linear(p_playback, map);
+}
+
+void AudioServer::set_playback_pitch_scale(Ref<AudioStreamPlayback> p_playback, float p_pitch_scale) {
+	ERR_FAIL_COND(p_playback.is_null());
+
+	AudioStreamPlaybackListNode *playback_node = _find_playback_list_node(p_playback);
+	if (!playback_node) {
+		return;
+	}
+
+	playback_node->pitch_scale.set(p_pitch_scale);
+}
+
+void AudioServer::set_playback_paused(Ref<AudioStreamPlayback> p_playback, bool p_paused) {
+	ERR_FAIL_COND(p_playback.is_null());
+
+	AudioStreamPlaybackListNode *playback_node = _find_playback_list_node(p_playback);
+	if (!playback_node) {
+		return;
+	}
+	if (!p_paused && playback_node->state == AudioStreamPlaybackListNode::PLAYING) {
+		return; // No-op.
+	}
+	if (p_paused && (playback_node->state == AudioStreamPlaybackListNode::PAUSED || playback_node->state == AudioStreamPlaybackListNode::FADE_OUT_TO_PAUSE)) {
+		return; // No-op.
+	}
+
+	AudioStreamPlaybackListNode::PlaybackState new_state, old_state;
+	do {
+		old_state = playback_node->state.load();
+		new_state = p_paused ? AudioStreamPlaybackListNode::FADE_OUT_TO_PAUSE : AudioStreamPlaybackListNode::PLAYING;
+	} while (!playback_node->state.compare_exchange_strong(old_state, new_state));
+}
+
+void AudioServer::set_playback_highshelf_params(Ref<AudioStreamPlayback> p_playback, float p_gain, float p_attenuation_cutoff_hz) {
+	ERR_FAIL_COND(p_playback.is_null());
+
+	AudioStreamPlaybackListNode *playback_node = _find_playback_list_node(p_playback);
+	if (!playback_node) {
+		return;
+	}
+
+	playback_node->attenuation_filter_cutoff_hz.set(p_attenuation_cutoff_hz);
+	playback_node->highshelf_gain.set(p_gain);
+}
+
+bool AudioServer::is_playback_active(Ref<AudioStreamPlayback> p_playback) {
+	ERR_FAIL_COND_V(p_playback.is_null(), false);
+
+	AudioStreamPlaybackListNode *playback_node = _find_playback_list_node(p_playback);
+	if (!playback_node) {
+		return false;
+	}
+
+	return playback_node->state.load() == AudioStreamPlaybackListNode::PLAYING;
+}
+
+float AudioServer::get_playback_position(Ref<AudioStreamPlayback> p_playback) {
+	ERR_FAIL_COND_V(p_playback.is_null(), 0);
+
+	AudioStreamPlaybackListNode *playback_node = _find_playback_list_node(p_playback);
+	if (!playback_node) {
+		return 0;
+	}
+
+	return playback_node->stream_playback->get_playback_position();
+}
+
+bool AudioServer::is_playback_paused(Ref<AudioStreamPlayback> p_playback) {
+	ERR_FAIL_COND_V(p_playback.is_null(), false);
+
+	AudioStreamPlaybackListNode *playback_node = _find_playback_list_node(p_playback);
+	if (!playback_node) {
+		return false;
+	}
+
+	return playback_node->state.load() == AudioStreamPlaybackListNode::PAUSED || playback_node->state.load() == AudioStreamPlaybackListNode::FADE_OUT_TO_PAUSE;
+}
+
+uint64_t AudioServer::get_mix_count() const {
+	return mix_count;
+}
+
+void AudioServer::notify_listener_changed() {
+	for (CallbackItem *ci : listener_changed_callback_list) {
+		ci->callback(ci->userdata);
+	}
+}
+
 void AudioServer::init_channels_and_buffers() {
 	channel_count = get_channel_count();
 	temp_buffer.resize(channel_count);
+	mix_buffer.resize(buffer_size + LOOKAHEAD_BUFFER_SIZE);
 
 	for (int i = 0; i < temp_buffer.size(); i++) {
 		temp_buffer.write[i].resize(buffer_size);
@@ -943,7 +1350,7 @@ void AudioServer::init() {
 	channel_disable_threshold_db = GLOBAL_DEF_RST("audio/buses/channel_disable_threshold_db", -60.0);
 	channel_disable_frames = float(GLOBAL_DEF_RST("audio/buses/channel_disable_time", 2.0)) * get_mix_rate();
 	ProjectSettings::get_singleton()->set_custom_property_info("audio/buses/channel_disable_time", PropertyInfo(Variant::FLOAT, "audio/buses/channel_disable_time", PROPERTY_HINT_RANGE, "0,5,0.01,or_greater"));
-	buffer_size = 1024; //hardcoded for now
+	buffer_size = 512; //hardcoded for now
 
 	init_channels_and_buffers();
 
@@ -1030,9 +1437,17 @@ void AudioServer::update() {
 	prof_time = 0;
 #endif
 
-	for (Set<CallbackItem>::Element *E = update_callbacks.front(); E; E = E->next()) {
-		E->get().callback(E->get().userdata);
+	for (CallbackItem *ci : update_callback_list) {
+		ci->callback(ci->userdata);
 	}
+	mix_callback_list.maybe_cleanup();
+	update_callback_list.maybe_cleanup();
+	listener_changed_callback_list.maybe_cleanup();
+	playback_list.maybe_cleanup();
+	for (AudioStreamPlaybackBusDetails *bus_details : bus_details_graveyard) {
+		bus_details_graveyard.erase(bus_details, [](AudioStreamPlaybackBusDetails *d) { delete d; });
+	}
+	bus_details_graveyard.maybe_cleanup();
 }
 
 void AudioServer::load_default_bus_layout() {
@@ -1098,40 +1513,49 @@ double AudioServer::get_time_since_last_mix() const {
 
 AudioServer *AudioServer::singleton = nullptr;
 
-void AudioServer::add_callback(AudioCallback p_callback, void *p_userdata) {
-	lock();
-	CallbackItem ci;
-	ci.callback = p_callback;
-	ci.userdata = p_userdata;
-	callbacks.insert(ci);
-	unlock();
+void AudioServer::add_update_callback(AudioCallback p_callback, void *p_userdata) {
+	CallbackItem *ci = new CallbackItem();
+	ci->callback = p_callback;
+	ci->userdata = p_userdata;
+	update_callback_list.insert(ci);
 }
 
-void AudioServer::remove_callback(AudioCallback p_callback, void *p_userdata) {
-	lock();
-	CallbackItem ci;
-	ci.callback = p_callback;
-	ci.userdata = p_userdata;
-	callbacks.erase(ci);
-	unlock();
+void AudioServer::remove_update_callback(AudioCallback p_callback, void *p_userdata) {
+	for (CallbackItem *ci : update_callback_list) {
+		if (ci->callback == p_callback && ci->userdata == p_userdata) {
+			update_callback_list.erase(ci, [](CallbackItem *c) { delete c; });
+		}
+	}
 }
 
-void AudioServer::add_update_callback(AudioCallback p_callback, void *p_userdata) {
-	lock();
-	CallbackItem ci;
-	ci.callback = p_callback;
-	ci.userdata = p_userdata;
-	update_callbacks.insert(ci);
-	unlock();
+void AudioServer::add_mix_callback(AudioCallback p_callback, void *p_userdata) {
+	CallbackItem *ci = new CallbackItem();
+	ci->callback = p_callback;
+	ci->userdata = p_userdata;
+	mix_callback_list.insert(ci);
 }
 
-void AudioServer::remove_update_callback(AudioCallback p_callback, void *p_userdata) {
-	lock();
-	CallbackItem ci;
-	ci.callback = p_callback;
-	ci.userdata = p_userdata;
-	update_callbacks.erase(ci);
-	unlock();
+void AudioServer::remove_mix_callback(AudioCallback p_callback, void *p_userdata) {
+	for (CallbackItem *ci : mix_callback_list) {
+		if (ci->callback == p_callback && ci->userdata == p_userdata) {
+			mix_callback_list.erase(ci, [](CallbackItem *c) { delete c; });
+		}
+	}
+}
+
+void AudioServer::add_listener_changed_callback(AudioCallback p_callback, void *p_userdata) {
+	CallbackItem *ci = new CallbackItem();
+	ci->callback = p_callback;
+	ci->userdata = p_userdata;
+	listener_changed_callback_list.insert(ci);
+}
+
+void AudioServer::remove_listener_changed_callback(AudioCallback p_callback, void *p_userdata) {
+	for (CallbackItem *ci : listener_changed_callback_list) {
+		if (ci->callback == p_callback && ci->userdata == p_userdata) {
+			listener_changed_callback_list.erase(ci, [](CallbackItem *c) { delete c; });
+		}
+	}
 }
 
 void AudioServer::set_bus_layout(const Ref<AudioBusLayout> &p_bus_layout) {

servers/audio_server.h

@@ -34,12 +34,17 @@
 #include "core/math/audio_frame.h"
 #include "core/object/class_db.h"
 #include "core/os/os.h"
+#include "core/templates/safe_list.h"
 #include "core/variant/variant.h"
 #include "servers/audio/audio_effect.h"
+#include "servers/audio/audio_filter_sw.h"
+
+#include <atomic>
 
 class AudioDriverDummy;
 class AudioStream;
 class AudioStreamSample;
+class AudioStreamPlayback;
 
 class AudioDriver {
 	static AudioDriver *singleton;
@@ -155,7 +160,10 @@ public:
 	};
 
 	enum {
-		AUDIO_DATA_INVALID_ID = -1
+		AUDIO_DATA_INVALID_ID = -1,
+		MAX_CHANNELS_PER_BUS = 4,
+		MAX_BUSES_PER_PLAYBACK = 6,
+		LOOKAHEAD_BUFFER_SIZE = 32,
 	};
 
 	typedef void (*AudioCallback)(void *p_userdata);
@@ -219,7 +227,43 @@ private:
 		int index_cache;
 	};
 
+	struct AudioStreamPlaybackBusDetails {
+		bool bus_active[MAX_BUSES_PER_PLAYBACK] = { false, false, false, false, false, false };
+		StringName bus[MAX_BUSES_PER_PLAYBACK];
+		AudioFrame volume[MAX_BUSES_PER_PLAYBACK][MAX_CHANNELS_PER_BUS];
+	};
+
+	struct AudioStreamPlaybackListNode {
+		enum PlaybackState {
+			PAUSED = 0, // Paused. Keep this stream playback around though so it can be restarted.
+			PLAYING = 1, // Playing. Fading may still be necessary if volume changes!
+			FADE_OUT_TO_PAUSE = 2, // About to pause.
+			FADE_OUT_TO_DELETION = 3, // About to stop.
+			AWAITING_DELETION = 4,
+		};
+		// If zero or positive, a place in the stream to seek to during the next mix.
+		SafeNumeric<float> setseek;
+		SafeNumeric<float> pitch_scale;
+		SafeNumeric<float> highshelf_gain;
+		SafeNumeric<float> attenuation_filter_cutoff_hz; // This isn't used unless highshelf_gain is nonzero.
+		AudioFilterSW::Processor filter_process[8];
+		// Updating this ref after the list node is created breaks consistency guarantees, don't do it!
+		Ref<AudioStreamPlayback> stream_playback;
+		// Playback state determines the fate of a particular AudioStreamListNode during the mix step. Must be atomically replaced.
+		std::atomic<PlaybackState> state = AWAITING_DELETION;
+		// This data should only ever be modified by an atomic replacement of the pointer.
+		std::atomic<AudioStreamPlaybackBusDetails *> bus_details = nullptr;
+		// Previous bus details should only be accessed on the audio thread.
+		AudioStreamPlaybackBusDetails *prev_bus_details = nullptr;
+		// The next few samples are stored here so we have some time to fade audio out if it ends abruptly at the beginning of the next mix.
+		AudioFrame lookahead[LOOKAHEAD_BUFFER_SIZE];
+	};
+
+	SafeList<AudioStreamPlaybackListNode *> playback_list;
+	SafeList<AudioStreamPlaybackBusDetails *> bus_details_graveyard;
+
 	Vector<Vector<AudioFrame>> temp_buffer; //temp_buffer for each level
+	Vector<AudioFrame> mix_buffer;
 	Vector<Bus *> buses;
 	Map<StringName, Bus *> bus_map;
 
@@ -230,18 +274,19 @@ private:
 	void init_channels_and_buffers();
 
 	void _mix_step();
+	void _mix_step_for_channel(AudioFrame *p_out_buf, AudioFrame *p_source_buf, AudioFrame p_vol_start, AudioFrame p_vol_final, float p_attenuation_filter_cutoff_hz, float p_highshelf_gain, AudioFilterSW::Processor *p_processor_l, AudioFilterSW::Processor *p_processor_r);
+
+	// Should only be called on the main thread.
+	AudioStreamPlaybackListNode *_find_playback_list_node(Ref<AudioStreamPlayback> p_playback);
 
 	struct CallbackItem {
 		AudioCallback callback;
 		void *userdata;
-
-		bool operator<(const CallbackItem &p_item) const {
-			return (callback == p_item.callback ? userdata < p_item.userdata : callback < p_item.callback);
-		}
 	};
 
-	Set<CallbackItem> callbacks;
-	Set<CallbackItem> update_callbacks;
+	SafeList<CallbackItem *> update_callback_list;
+	SafeList<CallbackItem *> mix_callback_list;
+	SafeList<CallbackItem *> listener_changed_callback_list;
 
 	friend class AudioDriver;
 	void _driver_process(int p_frames, int32_t *p_buffer);
@@ -319,6 +364,25 @@ public:
 	void set_playback_speed_scale(float p_scale);
 	float get_playback_speed_scale() const;
 
+	void start_playback_stream(Ref<AudioStreamPlayback> p_playback, StringName p_bus, Vector<AudioFrame> p_volume_db_vector, float p_start_time = 0);
+	void start_playback_stream(Ref<AudioStreamPlayback> p_playback, Map<StringName, Vector<AudioFrame>> p_bus_volumes, float p_start_time = 0);
+	void stop_playback_stream(Ref<AudioStreamPlayback> p_playback);
+
+	void set_playback_bus_exclusive(Ref<AudioStreamPlayback> p_playback, StringName p_bus, Vector<AudioFrame> p_volumes);
+	void set_playback_bus_volumes_linear(Ref<AudioStreamPlayback> p_playback, Map<StringName, Vector<AudioFrame>> p_bus_volumes);
+	void set_playback_all_bus_volumes_linear(Ref<AudioStreamPlayback> p_playback, Vector<AudioFrame> p_volumes);
+	void set_playback_pitch_scale(Ref<AudioStreamPlayback> p_playback, float p_pitch_scale);
+	void set_playback_paused(Ref<AudioStreamPlayback> p_playback, bool p_paused);
+	void set_playback_highshelf_params(Ref<AudioStreamPlayback> p_playback, float p_gain, float p_attenuation_cutoff_hz);
+
+	bool is_playback_active(Ref<AudioStreamPlayback> p_playback);
+	float get_playback_position(Ref<AudioStreamPlayback> p_playback);
+	bool is_playback_paused(Ref<AudioStreamPlayback> p_playback);
+
+	uint64_t get_mix_count() const;
+
+	void notify_listener_changed();
+
 	virtual void init();
 	virtual void finish();
 	virtual void update();
@@ -340,12 +404,15 @@ public:
 	virtual double get_time_to_next_mix() const;
 	virtual double get_time_since_last_mix() const;
 
-	void add_callback(AudioCallback p_callback, void *p_userdata);
-	void remove_callback(AudioCallback p_callback, void *p_userdata);
+	void add_listener_changed_callback(AudioCallback p_callback, void *p_userdata);
+	void remove_listener_changed_callback(AudioCallback p_callback, void *p_userdata);
 
 	void add_update_callback(AudioCallback p_callback, void *p_userdata);
 	void remove_update_callback(AudioCallback p_callback, void *p_userdata);
 
+	void add_mix_callback(AudioCallback p_callback, void *p_userdata);
+	void remove_mix_callback(AudioCallback p_callback, void *p_userdata);
+
 	void set_bus_layout(const Ref<AudioBusLayout> &p_bus_layout);
 	Ref<AudioBusLayout> generate_bus_layout() const;