I have the same problem on Big Sur however the problem has also popped up for me on Catalina and Mojave from time to time over the last few years. I tried disabling vp9 encoding as suggested but it didn't work for me. The solution I have used in the past that worked and seems to always work is to kill the "coreaudiod" process via "Activity Monitor", allow it to automatically relaunch and wait a few minutes. If you kill "coreaudiod" and then loose all system sound you just need to wait longer and it will return.