Sound volume icon (volume control widget) disappears from tray and Fn-F11/Fn-F12 keys do not work
Solution 1:
It seems that the issue was that plasma-pa package was missing.
I guess it got uninstalled for some reason.
Reinstalling it and restarting plasmashell fixes the problem:
sudo apt-get install plasma-pa
killall plasmashell
plasmashell &