No Sound on Thinkpad Running Ubuntu Linux

The other day sound on my Thinkpad stopped working (T60 Ubuntu Fiesty). I wasn't sure if it was some updates I installed (that required a reboot) or if something was blocking my sound driver (if you view a PDF using Evidence, for example, even after you close it the process will still be running and will block your sound card).

Rebooting didn't fix it, and I was sure all the sound modules were loaded correctly. I finally tracked it down -- somehow this "PCM" setting in alsamixer (which I'd never even seen before) was on "Mute". "MM" means "Mute" and "00" means "Not Muted", a setting which can be toggled by hitting "M". I have no idea how this got muted, but here's the fix: