I myself am quite new to Linux and I also use Debian (sarge). My sound card is a AC97. When I first installed Debian it worked fine, but today I decided to switch from Gnome to KDE. So I installed KDE and surprise...the sound card semmed not to work anymore (I must mention that previously I installed alsa).
The problem was after all, that the master channel was muted in the alsa mixer.
I didn't understand from your post if you have alsa installed or not, but if you don't I'd recommend you to install it (using either "apt-get install alsa-base alsa-utils" or System->Synaptic Package Manager). After you get it installed it will ask you to run alsa-conf and then, just to make sure, run "alsamixer" and adjust the channels properly.
Hope this helped. Cheers.