Add audioBitrate option for publishAudioTrack. (#124)

lib/src/options.dart

@@ -105,6 +105,12 @@ class VideoPublishOptions {
       '${runtimeType}(videoEncoding: ${videoEncoding}, simulcast: ${simulcast})';
 }
 
+class AudioPreset {
+  static const telephone = 12000;
+  static const speech = 20000;
+  static const music = 32000;
+}
+
 /// Options used when publishing audio.
 class AudioPublishOptions {
   /// Whether to enable DTX (Discontinuous Transmission) or not.
@@ -112,8 +118,12 @@ class AudioPublishOptions {
   /// Defaults to true.
   final bool dtx;
 
+  /// max audio bitrate
+  final int audioBitrate;
+
   const AudioPublishOptions({
     this.dtx = true,
+    this.audioBitrate = AudioPreset.speech,
   });
 
   @override

lib/src/participant/local.dart

@@ -64,6 +64,10 @@ class LocalParticipant extends Participant<LocalTrackPublication> {
 
     final transceiverInit = rtc.RTCRtpTransceiverInit(
       direction: rtc.TransceiverDirection.SendOnly,
+      sendEncodings: [
+        if (publishOptions.audioBitrate > 0)
+          rtc.RTCRtpEncoding(maxBitrate: publishOptions.audioBitrate),
+      ],
     );
     // addTransceiver cannot pass in a kind parameter due to a bug in flutter-webrtc (web)
     track.transceiver = await room.engine.publisher?.pc.addTransceiver(