7.1 not working please help!!!

Hello,
hey guys, hopefully someone can help... i have xp pro...i just bought a nice 7.1 channel stereo and reciever, everything sounds great but when i go into control panel and sounds and audio devices, everything works up to 5.1 channels but when i switch it to 7.1 channels, the sound is being distributed correctly but the music i'm playing slows down and plays alot slower....
if anyone knows why this is happening please reply
thanks