can't play video files (mp4, avi, mkv) with 5.1 audio tracks

I have two Macs, one with a mini-optical out and no audio through its mini-displayport, and the other that supports digital audio out through its mini-displayport. On either of my Macs, how can I play video files (mp4, avi, mkv) in 5.1 using Quicktime or iTunes or VLC? I've heard I need Perian. Is this true? If so, how do I configure it on either machine?

Note that when I connect the computer that supports audio through the mini-displayport to my receiver, my home theater receiver detects a PCM audio connection, but the audio I get out of my speakers is still stereo.

Both computers are running Snow Leopard. I haven't had the need to upgrade to Lion yet, but if the answer is that I need Lion do do this, I'd spend the $29.

Thanks for the help!