Big Sur does not retain sound preferences when it restarts. After buying a set of Edifier speakers and setting my Sound settings for them, I noticed that the settings were not stable for output and probably for input too, and when I installed the latest security upgrade, I watched carefully to see if it changed the sound settings. It did, switching output to the “external headphone” setting, which is connected to the crummy speakers in my display. I’m not sure what causes the change, but it’s a nuisance if you have to use different sound settings for different tasks. I sometimes find I have to use different Sound setups for different videoconference services or connections, and it’s annoying to find that Big Sur has changed them in a restart.