I had this problem, too.
Try running the sound card detector (if you have red hat) - somewhere in system tools or similar.
click "play test sound" - turn your speakers way up for a moment, like nearly full - you should just barely hear a sound (well, I did, with my ear up close to the speaker).
then click "yes", you heard it. then turn speakers down, then go to sound volume and put the volume option full, and the next option on the right almost full - t
hen try play an mp3 or whatever - it should work fine. muck around with other settings if need be.
hope that works.