great question...
That is the question of the hour sir...at least for me as well.
I also have a 5.1 speaker system that I am having a terrible time connecting to my iMac. Ok, well, I've figured out the connection part. If your speakers don't have an optical input, then you're going to need an external soundcard. If your speakers do have an optical input, you need a toslink adapter (
Amazon.com: Recoton Fiber Optic Toslink to 3.5mm Mini Adapter: Electronics ) and one of these,
Amazon.com: PCD Basics Fiber Optic Audio Cable - 6ft (1.8M): Electronics
If you are in the boat of not having an optical input, but maybe 3 1/8" jacks for center/front, surround speakers then you'll need to find a soundcard.
I just bought a "Hercules Muse XL Pocket LT3 and it works, does what it's supposed to do. You will have to configure your output in your MIDI settings from the default "built in" output to the USB output.
The problem I'm currently having is that I still can't figure out how to get iTunes to play out of all of the speakers. Everyone says that iTunes only outputs a 2.1 signal so "of course" I can't listen to music out of all speakers. Kind of defeats the purpose of having a 5.1 system...
On my PC, it can tell the difference. That is, when I watch movies, the 5.1 surround works as a surround system. When I listen to music, it basically outputs a "5 channel stereo" where I have sound in all of my speakers.
I have been trying to figure this out for about a week now and haven't had any luck.
If anyone here knows how to make iTunes play music out of all of my speakers, I would love to know about it!
Thanks!
If i connect one of the cables then 2 of the corresponding speakers work. 
