Did you happen to install pulseaudio?
Pulseaudio and Alsa DO NOT play well together!
I made the same mistake, thinking I needed it for something i installed, knowing damn well that it screwed up
my other machine running Jaunty with LXDE on it. I had to install another pulseaudio file to fix it.
Oh, the dependencies.....
Anyway, I was going to let it ride for now on my MINT 8/LXDE install knowing that Kendall is going to release 9 soon.
But since someone else had the problem I decided to give it a go.
I based the fix on Lubuntu where I'm running Rythembox and streaming internet radio without pulse! YEAH!
This may be a bit convoluted, and the files I removed/added may not all be necessary, but It worked for me.
So here goes.
Open synaptic and search for pulseaudio. Mark pulseaudio for complete removal.
However this does not remove everything. Mark everything that is installed having to do with pulse,
EXCEPT: libpuls0 and libpuls-mainloop-glib0
Then search for Alsa and install (if not installed),
then un-mute the volume control on the panel.
I restarted my machine at this point.
After a restart open Alsamixer.
PCM will be set at 0. Raise the bars up, and click on the speakers above
to turn then on. That should do it.
Hopefully I got all the file names right. (sometimes I can't read my own scribble).
I don't know if this will works for Gnome, but works here with LXDE.
I hope it works for you.