Add missing 0x00 type byte to outbound MumbleUDP.Audio packets

Wumble prepends Bytes[0_u8] before the protobuf fields when building
the plaintext for encryption. My encodeUDPAudio was missing this type
byte, so the server decrypted our packets and saw raw protobuf without
a message type indicator — silently dropping them.

Also added type-byte dispatch on receive side: 0x00 = audio, 0x01 = ping.

gumble/gumble/udp15.go

@@ -463,8 +463,9 @@ func (c *Client) WriteAudioUDP15(data []byte, final bool) (bool, error) {
 
 	frameNum := nextFrameNumber()
 
-	// Build MumbleUDP.Audio protobuf.
-	payload := encodeUDPAudio(0, frameNum, data, final)
+	// Build MumbleUDP.Audio: type byte 0x00 + protobuf.
+	// Matches wumble: Bytes[0_u8] + Protobuf.field(4, ...) + Protobuf.bytes(5, ...)
+	payload := append([]byte{0x00}, encodeUDPAudio(0, frameNum, data, final)...)
 
 	// Encrypt with 1.5 format.
 	encrypted, err := cs.encrypt15(payload)
@@ -512,6 +513,23 @@ func (c *Client) HandleUDPPacket15(packet []byte, pktNum uint64) {
 
 	log.Debug("UDP15 #%d: decrypt OK, plaintext_len=%d", pktNum, len(plaintext))
 
+	// Check type byte (0x00 = Audio, 0x01 = Ping)
+	if len(plaintext) < 1 {
+		return
+	}
+	msgType := plaintext[0]
+	plaintext = plaintext[1:]
+
+	if msgType == 0x01 {
+		// Ping response — just a timestamp, no action needed.
+		log.Debug("UDP15 #%d: ping response", pktNum)
+		return
+	}
+	if msgType != 0x00 {
+		log.Debug("UDP15 #%d: unknown message type 0x%02x", pktNum, msgType)
+		return
+	}
+
 	// Parse MumbleUDP.Audio protobuf.
 	session, frameNum, opusData, terminator := decodeUDPAudio(plaintext)