I have just bought an apple TV and am trying to get it to output to the optical port. The TV is plugged into a classroom projector at the moment so the onboard audio via the HDMI is quite poor. So i need the S/DIF output to send audio on another route..

my appletv is conected by hdmi to my tv

by optical to my receivers optical input

I mute my tv to about audio from 2 sources

I set my receivers active output source as the optical port in my case it call it BD because it's sony and they like blueray...

and it plays

some people repport that the hdmi have to have an active connection or the optical will be muted

when you troubleshoot you start with a simple setup rather then a complex one so ditch the projector maybe it's the cause connect a tv and the "another route" and test if it dont work then exchange the "another route" with a simple receiver and test again sooner or later you learn if your equipment is defect, incompatible or if your set it up worng.