Problem with sound

I have a Onkyo 5.1 setup with a Onkyo 7.2 channel receiver connected with HDMI to my new AppleTV.
I play back content from various sources like HBO and Netflix.
When AppleTV is set to Auto, it seems to output stereo only to the receiver.
If I select Dolby Digital all content shows up as Dolby Digital on the receiver, even Stereo, shich makes it sound really bad.
It works great on my AppleTV3 and my Bluray player.