Yesterday, i've meet the same probleme.
1 reboot my computer & sound work again.
This game is very great, but the real probleme is the sound. :
When I 'm alone on track, it's ok, but when the race begin, I can't hear my engine.
It's a real probleme for me. :(
When do you add a sound mixer (to adjuste my sound engine & copetitor's sound.)