According to page 10 on your
product's manual you can use an analog input from your onboard soundcard on your system with a 3.5mm analog cable running from your audio out(green) to the MP3 port on your MixAmp.
You may argue that you're not getting surround sound technology passed from your audio card to your headset MixAmp but in hindsight the headsets are designed with one driver per ear cup which means it's a stereo output. This eventually equates to providing virtual surround sound using a stereo source.