I've had intermittent troubles with applications not having sound because some other (often unfindable!) application was using the sound resource; sometimes using esd would allow more than one application to share the sound resources, sometimes not.

I went to play Frozen Bubbles (yes, another of those slow days...) and of course it's more fun with the monotonous music and the zippy sound effects. But F. B. was silent. I decided to poke around in the manual and I found a function

"alsactl restore" (without, need I add, the quotes)

which magically fixed the sound. This command causes alsa to attempt to restore the original state of the sound card.

So that's worth a try, if you're using alsa on your system.