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.