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 play text action is performed on all members of the conference.
Use either member_id or all the same way you did to start speaking text. If you initiated the play using member_id, calling the stop API with all will not stop the text from being spoken.