The Saga of HDMI and Disappointment, but victory lies ahead.
Back when I upgraded the Mac Mini to a model that came with HDMI, I was delighted. Finally I could play lossless multichannel audio on the same computer that controlled the TV and Stereo music. Previously I had to encode everything into DTS to fit it through the TOSLINK connection that was "required" on earlier models.
(Obviously I had non-Mac options. I had reasons other than music for this decision)
With great anticipation, I ripped Amused to Death into high res FLAC.
The tragedy was that (I thought at the time) the Mac Mini would see the TV in the HDMI chain and immediately downmix to stereo. Infuriating. The MIDI output would see all the channels, but when I'd play music, it would downmix to stereo. I was limited to 2-channel, 16 bit, 44.1 khz output with encoded digital audio to hear surround. Depressing.
By chance I learned that the problem was not the computer, but the software. iTunes and COG would force playback to stereo, but Songbird, Nightingale, and VLC would play multichannel music just fine.
Why?
I have no idea. I've been gleefully re-ripping all of my physical medial into multichannel flac. It's been a delight. Nightingale, which was created from the ashes of Songbird, works just fine for my needs.
One moment of sadness - the Bowie Young Americans DVD-A won't rip anymore. Depressing as hell. I can get to track four, and then blooey. I've tried cleaning the disk, using different programs, but no dice.
My advice: fiddle with everything if it's not working. There might be some dumb reason why it doesn't work, and you might happen upon a happy solution. I suspect there might be a setting in iTunes that will fix this, but I've tried everything in there and can't figure it out.