Tried restarting your PC?
Happens a lot with POS corporation built PC's.
If it's a software problem try restoring the PC to a time when it did work using the System Restore function. (See link below in case you're unfamiliar with the System Restore function.)
Use System Restore to Undo Changes if Problems Occur
As DJ said, try installing the drivers.
It's most likely that your sound is built in to the MoBo so find out what the sound driver that you need is, Google for it and dl it.