Try fiddling with some of the on/off buttons in the sound mixer. Go into preferences and make sure that you are seeing all the options.
You could make it easier and use console "alsamixer" do "man alsamixer" this way none of the possible channels and outputs are "hidden" by what gnome or KDE think u should see.
Do you know if your sound worked in any other linux distro?
Do you know what sound card it is?
Can you paste the output of: