sound issue

My sound capability disappears and I have to do a restart to get it back. In sound prefs the actual device for output leaves the list. Returns on restart. This has happened even during a zoom. In zoom not only can I not unmute to speak but my visual disappears as well.

What's going on? I'm using a late 2014 mac mini , latest OS Catalina, thunderbolt screen which is also sometimes flashing off/on. I don't know if that's related.

Thanks, Tzipora