Could be drivers. Do you have alsa installed? on the command line type
If it returns folders you have it otherwise get it at
http://www.alsa-project.org/ There are readmes on how to install it
If you've got it installed enter this in the command line:
The volumes are muted by default, unmute with m and turn up the volume
In the KDE control center go to the sound architecture (IIRC, if not it's a tab named hardware in the sound options) and choose ALSA as driver there. I think it should work immediately but you might need to logout and login again