i used to have this problem, and everyone told me it was a ground problem, i just got a new soundcard. and make sure every cord is pushed in all the way, cause mine hums even now if its not tightly in.
ive never had this problem. your av cord is probably fucked up. id just try and get a new one. mine is setup like this.. ive got a creative labs soundcard and then a Y av cord from that to my reciever on the cd input then all 6 speakers for 5.1 outta the pioneer reciever. sounds pretty god damn good IMO.
i got one cord that goes in my speaker port on my computer, and comes out into 2 cords, which go into a red and white port on the front of my stereo. i have a crystal fusion soundcard. check yur soundcard settings, and make sure your stereo is on aux