check the options menu and make sure all of your volume controls are set properly in SH3?
Update the sound card drivers?
surround sound? make sure your "center" knob is adjusted to true center... i have been playing for weeks cursing those GWX guys for making the engines so quiet you almost cant even hear them... and then i found that somehow my speaker's center position was adjusted way left!!!
:rotfl:
others will post further suggestions, thats all i have for you