音频轨的创建时序在Conductor::AddTracks()中

  rtc::scoped_refptr<webrtc::AudioTrackInterface> audio_track(peer_connection_factory_->CreateAudioTrack(kAudioLabel, peer_connection_factory_->CreateAudioSource(cricket::AudioOptions())));

通过代码我们可以看出，创建音频轨需要两个参数，第二个参数为通过PcFactory构建的音频源

rtc::scoped_refptr<AudioSourceInterface>
PeerConnectionFactory::CreateAudioSource(const cricket::AudioOptions& options) {RTC_DCHECK(signaling_thread_->IsCurrent());rtc::scoped_refptr<LocalAudioSource> source(LocalAudioSource::Create(&options));return source;
}

通过上面的代码我们可以看出，CreateAudioSource只是简单的创建了一个LocalAudioSource对象并返回

rtc::scoped_refptr<AudioTrackInterface> PeerConnectionFactory::CreateAudioTrack(const std::string& id,AudioSourceInterfa