fix: reset Opus decoder on packet loss and decode errors

Track per-user audio sequence numbers to detect UDP packet loss gaps.
When a sequence discontinuity is detected (loss, reorder, or burst gap),
reset the Opus decoder state to prevent permanent audio corruption.

Also reset the decoder when Decode() returns an error, instead of
leaving the decoder in a corrupted state that produces static/popping
for the remainder of the session.

This fixes the 'random static/popping from user b but no other clients
hear it' symptom, which occurs when one client experiences packet loss
affecting only its own per-user decoder instance.

gumble/gumble/handlers.go

@@ -113,13 +113,30 @@ func (c *Client) handleUDPTunnel(buffer []byte) error {
 	}
 
 	// Sequence
-	// TODO: use in jitter buffer
-	_, n = varint.Decode(buffer)
+	seq, n := varint.Decode(buffer)
 	if n <= 0 {
 		return errInvalidProtobuf
 	}
 	buffer = buffer[n:]
 
+	// Detect sequence gaps (packet loss) and reset the decoder to prevent
+	// permanent audio corruption from state desync.
+	// Mumble uses a monotonically increasing sequence that wraps at MaxInt32.
+	if user.audioSequenceValid {
+		// Only treat as discontinuity if the gap is small enough to be loss
+		// rather than a legitimate wrap-around or restart.
+		gap := seq - user.audioSequence
+		if gap > 1 && gap < 100 {
+			decoder.Reset()
+		} else if gap < 0 && gap > -100 {
+			// Reordered packet — reset to be safe, since the decoder
+			// state depends on correct frame ordering.
+			decoder.Reset()
+		}
+	}
+	user.audioSequence = seq
+	user.audioSequenceValid = true
+
 	// Length
 	length, n := varint.Decode(buffer)
 	if n <= 0 {
@@ -134,6 +151,8 @@ func (c *Client) handleUDPTunnel(buffer []byte) error {
 
 	pcm, err := decoder.Decode(buffer[:audioLength], AudioMaximumFrameSize)
 	if err != nil {
+		// Decode failure indicates corrupted decoder state; reset and drop.
+		decoder.Reset()
 		return err
 	}
 

gumble/gumble/user.go

@@ -53,6 +53,11 @@ type User struct {
 	client  *Client
 	decoder AudioDecoder
 
+	// audioSequence tracks the last UDP audio packet sequence number for
+	// this user, used to detect packet loss and reset the Opus decoder.
+	audioSequence      int64
+	audioSequenceValid bool
+
 	AudioSource *openal.Source
 	Boost       uint16
 	Volume      float32