Use Apple TV as airport express If I buy an Apple TV and connect my HiFi system to the optical out, will the sound go to the TV through the HDMI cable, and the through the optical cable? 
Or will the optical cable take precedent over the HDMI? 

Basically can I use the Apple TV as an Airport Express for my HiFi, when the TV is turned off, without plugging cables in and out?
The audio signal is sent to both the HDMI and the optical outputs at the same time, so yes, what you want to do is possible.