Fix: prevent concurrent Opus decoder access during session transfers

During session transfers, multiple handleInputTrackForSession goroutines
could call WriteMessage() concurrently on the shared inputSource, causing
Opus SILK decoder state corruption and assertion failures.

Added inputSourceMutex to serialize WriteMessage() calls and prevent
race conditions in both input and output audio paths.

audio.go

@@ -13,6 +13,8 @@ import (
 
 var (
 	audioMutex         sync.Mutex
+	setAudioTrackMutex sync.Mutex // Prevents concurrent setAudioTrack() calls
+	inputSourceMutex   sync.Mutex // Serializes WriteMessage() calls to input source
 	outputSource       audio.AudioSource
 	inputSource        atomic.Pointer[audio.AudioSource]
 	outputRelay        *audio.OutputRelay
@@ -130,6 +132,10 @@ func onWebRTCDisconnect() {
 }
 
 func setAudioTrack(audioTrack *webrtc.TrackLocalStaticSample) {
+	// Prevent concurrent calls to avoid creating multiple relays
+	setAudioTrackMutex.Lock()
+	defer setAudioTrackMutex.Unlock()
+
 	audioMutex.Lock()
 	currentAudioTrack = audioTrack
 	oldRelay := outputRelay
@@ -146,7 +152,6 @@ func setAudioTrack(audioTrack *webrtc.TrackLocalStaticSample) {
 		oldSource.Disconnect()
 	}
 
-	// Create new source and relay for the new track
 	audioMutex.Lock()
 	if currentAudioTrack != nil && audioOutputEnabled.Load() {
 		alsaDevice := "hw:1,0"
@@ -265,9 +270,12 @@ func handleInputTrackForSession(track *webrtc.TrackRemote) {
 			}
 		}
 
+		// Serialize WriteMessage() to prevent concurrent session handlers from corrupting Opus decoder
+		inputSourceMutex.Lock()
 		if err := (*source).WriteMessage(0, opusData); err != nil {
 			audioLogger.Warn().Err(err).Msg("failed to write audio message")
 			(*source).Disconnect()
 		}
+		inputSourceMutex.Unlock()
 	}
 }