Audio setup?

I have a Sony 5.1 Ch. Home Theater System hooked up to my iMac with a HDMI to Mini Displayport cable. My iMac is outputting surround sound just fine. I was just wondering what is the difference between using the Mini Displayport for surroud sound audio or using a Digital Optical cable through the headphone jack? After looking through the Apple support community, I haven't read of anyone using the Mini Display port to output surround sound.