The simple things are what should be checked first. Are the speakers plugged into the right jacks in the sound card? Is the volume muted in Windows? Is there a headphone jack on the front of the computer as well as the jacks on the back? If so test that too.

There's an option in the BIOS sometimes to disable the onboard sound/video/network cards. Those are used if you have a separate card you want to use instead. Might want to check that.