The DisplayPort connector on these iMacs is capable (through an
adaption of the display port plug to HDMI) of carrying up to 24bit,
96kHz, 8 channels of linear pcm audio.
Griffin technologies provide an adaptor to adapt the display port
connection to HDMI, and once you plug this in, and then hook up a
receiver on the other end of the HDMI cable.
http://www.griffintechnology.com/products/video-display-converter
Now, HDMI is driven by an EDID (a descriptor that describes the
capabilities of a given device). So, what you will see in AMS (Audio
MIDI Setup) ***once you plug a receiver in*** is the capabilities of
that receiver. Some of the older HDMI receivers are only able to do 2
channels of linear pcm audio over HDMI, so that is all you will see.
If your receiver can do mutli-channel linear pcm over HDMI, then AMS
will show that to you.
Once this is there, this then behaves like any audio device you have
plugged in to your Mac. That is, any application (iTunes, etc), any
game, can output audio to that receiver up to the number of channels
it supports.
So, I think this is the one cable solution you are looking for.
Bill