sounds like a driver issue.. try the latest drivers off the manufacturer website.
also how many audio jacks does your computer have?
if it just has 3 and you're posiive it supports 5.1
chances are you have to Change what function the jacks are in the driver level..
as there are three default jacks. One is Mic in, line in and Audio out
if it's not 5.1....
Get a 6 channel soundcard