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Ok my bad
ATV2 supports 5.1 ac3 ONLY if you don't use the 1/8 stereo jack as your sound output.
Sorry there's no help on this
Umm
There is no 1/8" stereo jack on the AppleTV 2
There is a toslink optical digital audio output that can send either stereo digital output or pass thru 5.1 ac-3 Dolby digital.
There are not any analog connections on the AppleTV 2!!!!
Did you try to stick a analog 1/8" jack into the optical port?
Sounds like it from one of your posts on another thread.