When using an iOS device and the Voice-Processing I/O unit is active, you cannot set the volume of the iOS device to 0. For more information, see the Apple developer documentation on AVAudioSessionCategoryPlayAndRecord.

A possible workaround for this functionality when you are using Vivox on iOS is to observe changes in AVAudioSession.outputVolume that are being set by the user and then mute your game audio output based on the observed value. For example implementations of this workaround, refer to How to: Observe the iOS volume level to programmatically mute game audio in the Unity Support Knowledge Base.