NVidia Sound Output Device Missing
You have already solved your problem.
Since you have not identified the source of your problem, I am posting this answer just in case it reappears in the future, and it is due to a similar problem I had, which took me quite some effort to solve.
The problem is described in Only "Dummy output" (sound) in Ubuntu 20.04 after reboot - Broken driver / module . alsa force-reload did not solve it.
The cause of the problem was that, after each kernel update, the corresponding drivers were not updated.
The accepted answer shows how to identify and solve the issue.
Avoid missing kernel linux-modules-extra-XX-generic when updating kernel shows how to get rid from the problem in the future once and for all.
If you find your problem reappears, and you can link it to a kernel update, you may find the info in the linked questions useful.
For anyone else who's having this problem, and the fix above didn't work: This often works;
sudo apt update
sudo apt-get install pavucontrol -y
Currently tested on:
- Ubuntu 20.04
And other Linux Distributions
- Arch Linux
- Debian
Hope this can be of any help to someone in the future!