I've got the same problem, except, I've never had sound to begin with. When the MB sound didn't work I installed an old sound card. Still no sound.
Any ideas??
I've been using linux for more than 6 months now with no major problems up until a few weeks ago. I installed some of the automatic updates and BAM (well not really bam, because I couldn't hear it..) I lost all sound. It's as if linux is perpetually on mute, except it says it's on full volume. Everything else seems to be working fine.

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