Sorry for the subject not-so-specific but I hope this message can start a first iteration to better focus my problem.
Sounds roughly similar to a problem I've been having. I tracked it down to Timidity grabbing the sound card. I don't know enough to make it fixed permanently, but as a temporary fix after a reboot when the problem manifests, I do a
ps ax | grep timidity
and then kill the Timidity process. Then my sound card gets recognized, and I have audio until my next reboot.