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.