I've managed to suss out a workaround on my AV receiver so that I can get audio through HDMI without needing the optical output from the Apple TV. It isn't very intuitive though and means I'd need the TV on, even if all I was doing was listening to music from the Apple TV.
The parents have an Apple TV3 which I thought was good. Just picked up the 4 for myself and I'm really impressed, so is there any must have apps that I should install?