If you are doing an Interview session with a participant using a Mac and they are not able to toggle the microphone, this is likely caused by the not having the proper microphone permissions set in macOS (See this help article).
But it can also be caused by them accidentally blocking permissions in the browser during the start of the session:
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.