usb doesn't carry 5.1
If the headset itself only uses usb, that must mean it has a dolby processor that connects to it that utilizes either hdmi or optical to get surround sound. You have to buy a toslink adapter for the optical cable to get it to hook up to the dolby headphones processor. If you'e on a mac pro and its audio card has built in fiber optic port, then use that. Then again if the audio card in the computer already has an optical out, it already probably does dolby headphone.