Good day.
You can mute remote (playing) audio on client side using WebSDK function
Stream.muteRemoteAudio() as we recommended in
this topic. Then you can unmute remote audio with function
Stream.unmuteRemoteAudio().
Remote (playing) video cannot be muted on client side.
Publishing audio and video can be muted on publisher side using
Stream.muteAudio() and
Stream.muteVideo() correspondingly.
If you wish to mute/unmute stream on server using REST API, it is not possible.