How do I get iTunes to play my HD video in 5.1?

I have a brand new Mac Mini and an exoense Bose surround sound system (Lifestyle V20). The Mini is plugged in via HDMI and in the System Preferences in OSX it knows what Bose I have. In the Midi settings app, it refuses to allow me to set the HDMI has having 5.1 channels. That's the first problem.

Secondly, is there a way to tell iTunes to always play the mutichannel (AC3) track. It seems idiotic to me that you buy a 1080HD video from iTunes, and you have to change what track you want on every video as you start to play it.

Thirdly, iTunes craps out half the time when I try to change the track to the AC3 one. It starts to play, and I the screen blanks, then flickers back on. I change the track to the AC3 one and the screen goes off again and eventually comes back on then the video plays a bit without sound then everything goes black.

I have to say, considering how user friendly Apple usually is, this is absurdly frustrating to set up. I know the Bose works because we also have our cable box plugged in via HDMI and it works perfectly fine.

Please, does anyone have any helpful suggestions?

Thanks