If a string is specified, can be loopback or loopbackWithMute. Specifying a loopback device will capture system audio, and is currently only supported on Windows. If a WebFrameMain is specified, will capture audio from that frame.
If audio is a WebFrameMain and this is set to true, then local playback of audio will not be muted (e.g. using MediaRecorder to record WebFrameMain with this flag set to true will allow audio to pass through to the speakers while recording). Default is false.