Sound card has disappeared from "Sound Settings" dialog

For me, the problem appears to have been corrupted configuration files in my home directory. I followed just the first part of the instructions from this sound troubleshooting page -- in particular steps 1A and 1B -- and that appeared to clear the problem for me. At this time, my sound is working as expected again.

I'd suggest the following, basically step 1B:

sudo killall pulseaudio
rm -r ~/.config/pulse/*
rm -r ~/.pulse*
sudo shutdown -h now

Let the computer sit quiet for 10 seconds or so, and then reboot.

I would not recommend following the remainder of the procedure on that page, unless you really know what you're doing, or are prepared to have to reinstall your system from scratch. It's very heavy handed.

This command restart all alsamixer config :

alsactl restore