5.1 surround question?

I am running windows 7 and have 5.1 speakers configured and working

the problem i am having is during watching DVDs the sound will crackle randomly but not offten. high and low power usages dont seem to make a difference either.

also when i play games – especially fallout new Vegas when there is a lot of stuff happening on the screen the game lags and i get a lot of the crackling sound its VERY annoying!!!!

Anybody familiar with this issue or if you think you have a solution please let me know

Thanks