I'm running Kubuntu 18.04. This morning I woke up to discover that sound wasn't working at all. Even doing aplay on a simple .wav file produced no sound. The aplay test is the lowest-level one I know. alsamixer shows only SP/DIF. I've ruled out hardware problems, since the sound is there when I switch to a different user.
My guess is that the sound is somehow being muted. There should be a loudspeaker icon among the group of small icons at the right end of the taskbar, and it's not there.
How can I get my sound back?