[nickr]

If you read back a few posts it is a receiver.

Silmaril -

2) All multichannel audio (all flavors of AC3 and DTS; 5.1 AAC) is downmixed to 2.0 if decoded onboard rather than bitstreamed.

if your computer is decoding a 5.1 stream on board, it has some choices:

1. stereo LPCM via spdif or HDMI
2. stereo analogue via two analogue ouptuts (possibly one 3.5mm stereo plug like you have on your mp3 player)
3. 5.1 via a soundcard with 6 analogue outputs, one per channel, with 6 analogue connections to your receiver (possibly 3 3.5mm plugs)
4. 5.1 LPCM via HDMI (not spdif) to your receiver.

Which are you wanting to achieve? (And what you can achieve depends on your hardware, OS and drivers.)

Oh and there are receivers that accept aac streams, although whether they do 7.1 is unknown to me.