You'll need a soundcard that has individual outputs for the surround channels. You'll feed the speakers in the back with surround L+R and your front speakers with L+R. If your soundcard only has analogue outputs you'll need to convert the signal to optical with an analogue to digital converter(ADC).
Living room: Kii Three/BXT with Control.
Den: Tannoy Precision 8 iDP with TS112 iDP subwoofer.
In the cupboard, waiting for a sibling: 1st gen. Phantom Silver running DOS1
My Phantom Voyage
Den: Tannoy Precision 8 iDP with TS112 iDP subwoofer.
In the cupboard, waiting for a sibling: 1st gen. Phantom Silver running DOS1
My Phantom Voyage