A couple of times I’ve encountered problems with the sound card on Debian boxes. Some notes I’ve found that may help others having difficulties:

• Check the permissions on /dev/dsp, and make sure that the relevant user(s) are in the audio group in /etc/group. You may need to log out & back in again for this to take effect.
• If using the alsa sound daemon, install the alsa-utils package and run alsaconf. This should pick up your sound card. You may need to run alsamixer to check the volume levels are correctly set afterwards.
• Blindingly Obvious (but easy to overlook) Note: make sure that
  the volume control on xmms (or other media player of choice), and on your headphones/speakers is sufficiently up. (I once spent ages prodding at a user’s sound config before realising that the headphones had their own volume control which I hadn’t noticed, and it was all the way down.)
• With KDE, I found recently that to get sound running it was necessary to run the KDE sound control utility, turn off KDE’s sound management and click Apply, close the utility down, reopen it, turn the sound management back on, & click Apply again. Then it worked.

Original post by Juliet Kemp