I don't know what happened, I fear black magic, but when I booted up alsa wasn't working, I don't know what I did, the last major modification I made was the install and uninstall of a large set of fonts for most of the major southeast asian languages, which in turn didn't allow me to run X untill I uninstalled them, but that's it. I have to say running arch is painful, but in stride with all the random fires I must put out, my knowledge grows tremendously.
did you see any messages to suggest problem with ALSA packages, or just not getting sound output (ALSA possibly working but muted somewhere)?
start withcat /proc/asound/cards
andaplay -l
to see if sound hardware is detected
then tryalsamixer
If you get to a mixer, check if anything has been muted or volume set to 0
if you get error messages they should indicate where to look further
If none of that works out, use thiswget http://www.alsa-project.org/alsa-info.sh -O alsa-info.sh && bash alsa-info.sh
You can then post the link/text file information on here or arch forum for folks to look into this further for you