^ Maybe you mean static? Or hiss? How high is your volume (e.g. 1 o'clock, 12 o'clock) set to?
If your PC's soundcard has "no Dolby encoding", are you sure that it can send out unprocessed surround sound signals? You can't just plug a MixAmp into any PC and expect it to work. (Nameless's thread probably has much more information about PC gaming)
Also: MixAmp's USB connection is not for surround sound at all.