That's pretty unusual but you need the motherboard driver CD to fix it. Enter the make and model of the motherboard in a Google search and you should be able to track it down. Alternatively, post it back here and someone will know.
my driver is kinda missing a couple of things. and i just figured out that it was missing ports com and lpt. it just happened and now my speakers are out. can anyone tell me where i can get one of these things