"somehow 5.1 sound" is also called "Dolby Surround (R)". The nForce sound chipset has SPDIF in/out jacks for 5.1. Only pre-mastered 5.1 streams can be played via the SPDIF out jack to a proprietary decoder. Encoding is also proprietary.
maybe an idea for an external, 6 inputs, creating that stream and sending it to the card to let it disassemble and send it to the 6 outputs. 2 Cons: - might involve some patents of dolby labs and so costs a fee
- that stream typical is lossy encoded as AC3 so the musical data is
changed which is to avoid. I don't know if there is a layer above or below to speak direct to the card but I guess this stream is sent to a decoder chip and than direct to the dacs.
An xbox is a cool idea although I wanted to give as few as possible money to M$. But I will think about it...
Cheers,
Malte