setAudioChannelCallback method
Implementation
void setAudioChannelCallback(String name, Function f)
{
  if(!_audioChannelCallbacks.containsKey(name)) {
    _audioChannelCallbacks[name] = AudioCallbackAndroidType.noBuffer();
  }
  _audioChannelCallbacks[name]!.callbacks.add(f);
  Map<String, String> msg = {
    MessageTag.MESSAGE_TYPE: MessageType.SET_AUDIO_CHANNEL_CALLBACK, MessageTag.CHANNEL_NAME: name
  };
  _sendMessage(msg);
}