Am I outputting true Surround Sound from my iMac/MacBook Pro?

I have a 27 " 2.66 GHz Intel Core i5 iMac (late 2009) and a 15" 2.2 Intel Core i7 GHz MacBook (early 2011)

I have my Sony 5.1 Ch. Home Theater System connect by a Mini Displayport to HDMI cable. The HDMI is plugged into the HDMI input of my reciever and the Mini Displayport into the Mini Displayport on my Mac. I went into the Audio MIDI setup and was able to change the output to Multi-channel 5.1 Surround and tested each speaker with the test tone. Everything works fine. When I play DVDs in the DVD Player it sounds like the surround sound is working perfectly. All the speakers work.

I've read on multi sites and forums that you need a sound card, or USB, or an Optical (TOSlink) cable to output surround so. No one mentioned anything about the Mini Displayport on the Mac. So I guess what I'm trying to find out is if I'm actually hearing true 5.1 Channel Surround Sound?