If your Interview participant can't toggle the microphone, this is likely caused by the not having the proper permissions set up for their microphone in the OS (See this help article).
But it can also be caused by them accidentally blocking permission during the start of the session. See picture.
This will manifest in the UI as if the microphone is not selectable. (We are working to improve this).
The solution is to click the red-crossed microphone symbol in the URL input field of their browser and allow access.