Apple TV>onkyo tx-sr576>TV - why doesn't it work?

Hi all.

Just tried setting up my new ATV, but try as I might, I can't get it to work when I route the hdmi cable through the receiver. It works fine when going just to the TV, but I only use the TV as a monitor so I get no sound doing it that way, plus I have the hdmi hard wired in the wall so no chance of running two cables to the TV! Any ideas how to resolve? I tried updating the ATV software but no joy!