As QuickTime will play Surround Sound, how do you output surround sound from the Mac; is it through the Digital Out?
If so, I assume that means the iMacs - which don't have digital out - cannot output surround sound from QuickTime, only a stereo mixdown. . . ?
from QuickTime Player Help Create surround audio
Create a rich multimedia experience for your customers by adding multi-channel audio to your movie. If some of your customers don
QT 7 can contain up to (i think) 24 tracks out audio. This audio can be compressed (aac, other) or uncompressed (up to 192/24bit), and can be assigned to a output (L, C, R, RR, LR...) or to a language track (Eng, FR, GR..). To get the 5.1 audio out of the mac you need a audio interface (like the m-audio firewire 410) that has 6 or more analog audio outputs. QT7 can not use common 5.1 "stream" formats like Dolby Digital or DTS (this should be coming soon QT8?) so using the optical out to play QT audio will down mix 5.1 to 2.0 LPCM 44.1 or 48K audio.
For demo 5.1 files I use the apple QT trailer site. Most of the HD trailers are in 5.1 aac. download one at 480P, 720P, or 1080P, and look at the movie's settings under the audio tab.