No sound sources (input or output) detected in Sound preferences. All disappeared.

I saw the last convo similar to this "Internal speaker is not showing in system preferences" is locked to replies, so I wanted to post that I am having this same problem. 2016 MacBook running Big Sur - nothing is plugged into the headphone jack.

I notice that this re-occurs most times AFTER I have unplugged HDMI or headphones. For example, I watched a movie last night on the laptop, plugged into an external HDMI monitor. I unplugged the HDMI cable this morning and went to watch something else, and no there are no, ZERO, none, sound outputs showing in my Sound preferences. When there should be internal speakers, Mixlr, and probably some other choices depending whatever's been installed lately.

A restart fixes this but it is very annoying as sometimes it happens in the middle of the workday - I'll watch a webinar with my headphones, then unplug them and get ready for a Zoom call, only to find after I've unplugged my headphones I've lost ALL sound outputs and input devices. My Sound panel is totally blank. It will take a few minutes to restart, and then I'll be late to my call or have to leave and rejoin. (Video is fine, wifi fine, etc.)

Screenshots of what is happening are attached. So, obviously this is not just an issue of "I left headphones or something else plugged in accidentally" or "I chose Mixlr SoundLink by accident and forgot to unchoose it" but my built-in mic and speakers (as well as ANY possible sound sources) have actually disappeared from existence in the eyes of this Mac. Welcome all of your advice or observations. Thanks.