shift produces unnatural sound

hello,

I have NFS shift installed in my system.recently when I was playing the game suddenly it produced very bad sound like an old TV,when I restarted the game it's sound was good but again it created some problem after playing for sometime.What may be the problem?

System specifications
As Rock G31 MVS motherboard
E7400 2.8Ghz
4 gb ram
Palit 9600 Gt graphics card
Creative 5.1 Vx sound card