When I change audio unit from VPIO(VoiceProcessingIO) to remoteIO, the volume was reduced and can not restore to normal level. There's a workaround by this post: h ttps://trac.pjsip.org/repos/ticket/1697 I use the workaround in my code and according flowing steps: stop and destory all the running IO AudioUnit (remoteIO and VPIO) run workaround code [[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryRecord error:nil]; [[AVAudioSession sharedInstance] setActive:NO error:nil]; [[AVAudioSession sharedInstance] setActive:YES error:nil]; [[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayAndRecord error:nil]; 3. setup remoteIO and run. and it work perfectly! The volume of remoteIO was go to normally. But sadness happens on iOS 14, the workaround is not work at all.