Thunderbolt Display speaker keeps disappearing after Big Sur update

Hi

Setup is Mac mini 2018 and 2 x Apple thunderbolt displays

After I've updated to Big Sur, immediately following reboot the audio options exists as they should i.e. in my case Mac mini speaker, Apple TV, display 1 and display 2. After "a while", however, the two displays disappear as audio output options (and sound reroutes from the chosen display to the built-in speak of the Mac mini. If I re-boot the Mac, everything goes back to working for a while (hours) but all of a sudden same thing, both display speakers disappear in the audio output options (the displays themselves continue to work as displays). At first I thought it had to do with the new auto switch to AirPods, i.e. once the Mac had switched it wouldn't go back to the monitor speakers. That may be part of the problem but not the whole problem. At my wits end with this. Anybody had similar experience or able to offer any advice (other than reboot which I've now been doing more in the last week than in the last year). thanks for any advice.