Need Help

I'm having sound issues on my pc and I am not sure how to track down the issue.
I have a custom built pc:
Windows Vista 64 is my os, I have an Auzentech X-Fi Forte sound card, I am also using a logitech 5500 THX certified 5.1 surround sound speaker system. IF more details are needed just ask. My issue is that I'm having problems with muffled sounds. When watching a dvd, bluray or either a movie on itunes, some the sounds and voices will be mufffled while all other sound effects and background noises come through just fine. I also have an issue with some video streams won't come through with any noise at all, while others come through just fine.