I used to have a problem in Amarok where if Amarok crashed, it would mute the sound without my noticing. Thankfully I remembered to check this in Kaffeine, but that was not the problem.
I don't mess with the sound settings often, so I had a fun time trying to find them (turns out they're not in your system or settings menu... try looking for KMix in the Multimedia menu instead.)
I log on to the freenode IRC channel #kubuntu, and a couple nice folks there had some suggestions for me.
I was told to pay special attention to KMix lines: master, master mono, PCM. These were all fine (at or near the top).
(You can also open alsamixer via the command line)
alsamixerI was then advised to restart ALSA.
sudo /etc/init.d/alsa-utils restartThis did not work. I tried closing all my programs which might be using sound, and running the command again. This did not work either.
I ran the command, and then rebooted. This works! Now I have my sound back.