have had an apple TV for over a year and it's worked as follows;
apple TV (HDMI to TV and optical out to receiver)....music and movie playback no issues (2 channel and 5.1).
changed things up recently and added an outboard DAC (Musical Fidelity V-DAC) before the pre-amp (2 channel only).
I get sound for movie playback, browsing movie trailers and music videos off the itunes store, but NO SOUND when playing back music files stored on the apple TV and when browsing the iTunes music store (30 second previews). The "click" sounds for navigating through the AppleTV menus is working, but no music playback works.
When I run the digital out back directly to my DD receiver, everything works.
When I use the AppleTV as an AirTunes terminal to the DAC, music plays back with no issues.
I'd like to browse my music catalogue on screen and not with a laptop or ipod Touch.
Because my receiver is so old (circa late 90's), the 2 channel music playback through the outboard DAC is definitely better than using the DAC in the receiver. I use the appleTV more for music than movie playback. I only use the AppleTV for the odd movie rental and won't mind the cable swapping for the one movie every two months to get 5.1 back.
To the OP, did this solve the problem? I am very interested in your experience. I am contemplating switching from my Wadia iTransport to an ATV, and before I do, I want to make sure that this is going to work.