It's either CPU or you need to update your sound card drivers. Refer to your manuals.
I highly doubt it is EITHER of these - it is only ONE sound (channel?), typically the engine of the buggy while driving (you let go of the accelerator and the engine sound STOPS completely, and when you tap the accelerator the engine noise resumes).
I have also noticed the soundtrack thing, it's kinda weird, I'm not sure what's up with that, I just assumed maybe I didn't complete the scripted event fast enough or something, so it replayed a piece etc.... ah the little things that pull us back into reality. It's like watching a movie in the theatre and every 5 minutes or so they flip on the lights again and everyone in the theatre points and you and goes "Ahhhhhhhhhhh" then goes dark again and back to the game.. yeah I HATE that feeling of being pulled out of the game

... heh, GL, I hope they fix this in the infamous approaching "patch".