Games sound like they are underwater

[Melodi]

I'm good with WinXP but I have no clue about WinXP Media Center Edition
I have my friend's kid's computer who says when he plays music and his games they sound like they are underwater. When they first got the PC it did not sound like this. They have an HP Media Center m470n, 3gig P4 processor, 512 RAM (and tons of games loaded). They bought it about a year ago for gaming.
There is a scsi raid controller
There are several different cards, seems like I'm going to have to read up on Media Software soon, but umm they'd like their computer back in this decade
Does anyone have a clue where I should start. I don't even think I have the right kind of speakers to plug into it.
Thanks
I got their speakers now and they are harmon/kardon speakers they have two green 'out' plugs. The back of the computer has the tradition onboard sound card with the three small plugs, the pink, blue and green. However there are two more cards installed, raid cards I believe with AV plugs but nothing on these speakers would plug into them. I can't see where they would be able to plug in these two small green 'out' ends
They play Halo and what I know about 'gaming' would not fill an ant's back.
Thanks