maybe a better way?: you might be able to connect the gfx card via HDMI to the receiver and have it count as a more or less phantom display by sending only audio.
http://www.overclock.net/t/1212983/eyefinity-questions-with-a-hdmi-receiver
I wasn't aware that one could actually do this

, but I guess it's possible now.
Assuming you have an extra HDMI output and 1 HDMI cable, you can get uncompressed 5.1 audio this way <3.
Otherwise, you can't send uncompressed 5.1 over optical, so you will need to compress it to DD 5.1 or DTS.
http://www.tomshardware.com/forum/302145-28-wont-play-optical-spdif-output
http://www.tomshardware.com/forum/342112-31-motherboard-audio-receiver
the game needs to support dolby DTS/DD 5.1, or you need some way to encode it into that format. Otherwise the game's probably just outputting PCM and clipping the extra channels. Some of the newer games are going the dolby route, but i'm not sure if it's that common (derp: I can't believe I forgot to realize that many games don't do dolby audio).
http://hardforum.com/showthread.php?t=1708835
Your realtek alc892 should have an option to encode sound as Dolby Digital Live before outputting it via optical [see hardforum link for pictures]. You might be able to find it in windows, though I doubt it [playback devices > device > advanced]
Link to the asrock z68 extreme4 download site, if you need to download the realtek HD audio manager (win 7 64-bit)
http://www.asrock.com/mb/overview.asp?cat=Download&os=Win764&Model=Z68%20Extreme4
[SIZE=-2]otherwise if the above doesn't work, creative happens to have software that can do this on specific cards (note the xtremeAUDIO is not included since it technically uses audigy chipsets)
http://us.store.creative.com/Dolby-Digital-Live-and-DTS-Connect/M/B006GK76QE.htm
the cards supported are:
Sound Blaster X-Fi Elite Pro
Sound Blaster X-Fi Platinum
Sound Blaster X-Fi
Sound Blaster X-Fi Fatal1ty
Sound Blaster X-Fi Platinum Fatal1ty Champion Series
Sound Blaster X-Fi XtremeMusic
Sound Blaster X-Fi XtremeGamer Fatal1ty® Pro
Sound Blaster X-Fi XtremeGamer
Sound Blaster X-Fi Titanium
Sound Blaster X-Fi Titanium Fatal1ty Professional Series
Sound Blaster X-Fi Titanium Fatal1ty Champion Series
x-fi xtremegamer sb0770 $28 (the half height xtremegamers don't have optical hah)
http://www.amazon.com/Genuine-Creati.../dp/B0093L6SCG[/SIZE]