iOS 使用AUGraph录音同时播放（并转码成Mp3）
https://www.jianshu.com/p/bcc2fb23c941
https://github.com/JIANHUI2015/RemoteIODemo
https://www.jianshu.com/p/9b8edca8d7c8
iOS 录音实现
https://developer.apple.com/library/content/samplecode/iOSMultichannelMixerTest/Introduction/Intro.html
DEMO
Using an AUGraph with the Multi-Channel Mixer and Remote I/O Audio Unit
https://justinyangjing.github.io/2017/04/19/iOS音频编程之混音/