A friend of mine has a Sound Blaster Audigy Platinum eX, and *thought* he had the latest drivers. His problem was that he had sound for everything, EXCEPT the car he was driving. He could hear other cars, just not his own.
What to do? DRIVERS DRIVERS DRIVERS!! Really can't say this enough for this game!
After updating the drivers to the latest available (which arn't very new... 2003 sometime!) everything was sweet once again...