[Jake]

I am pretty sure you can change the audio stream on-the-fly in the ATV. Too late for me to test it but I had an M4V file with 5.1 AC3 and AAC stereo. It streamed via wifi which I assume you are talking about.

Just occurred to me are you talking about the ATV2? I was.

 

[Jake]

I must be confusing it with my WD TV Live (which can do that). I could have sworn I saw an audio stream option in iTunes.