When upgrading from Ubuntu 20 to 21 this spring, my father and I both had the rear audio line out on our computers stop working. As we both had the same problem, I figured I would write it up in case anyone else was affected.
sudo lsof /dev/snd/*in a console shows Timidity holding open some files.
timidityprocess makes the audio device show up again.
sudo apt remove timidity.
Somewhat surprisingly, this does not seem to affect my ability to play back Midi files. I assume my music program is using Fluidsynth, which is a separate system? Nonetheless, a strange bug caused by a misconfiguration somewhere.