The member_id attribute that’s passed in the URL can be a member_id, a comma-separated list of member IDs on which this operation will be performed, or the string all. In the latter case, the audio stops playing for all members of the conference.
Use either member_id or all the same way you did to start playing audio to a member. If you initiated the play using member_id, calling the stop API with all will not stop the audio.