Fix AudioServer::start_playback_stream does not iterate through given p_bus_volumes

(cherry picked from commit e81887f996a110081c288be42e30c3dc05e913ef)

servers/audio_server.cpp

@@ -1147,6 +1147,7 @@ void AudioServer::start_playback_stream(Ref<AudioStreamPlayback> p_playback, Has
 		for (int channel_idx = 0; channel_idx < MAX_CHANNELS_PER_BUS; channel_idx++) {
 			new_bus_details->volume[idx][channel_idx] = pair.value[channel_idx];
 		}
+		idx++;
 	}
 	playback_node->bus_details = new_bus_details;
 	playback_node->prev_bus_details = new AudioStreamPlaybackBusDetails();