AV Receiver is Denon AVR1911, Sony TV and Sky+HD box
I have Optical Coaxial leads and HDMI cable from HD box to TV.
Can't get sound whilst watching TV through Sky+HD?

The fiber optic connection on your TV is likely an OUTPUT and not an INPUT. This only passes 2.0 sound out unless you are using the internal tuner for basic cable or antenna.

The best method to connect this equipment would be the following:

HD box to A/V Receiver via HDMI.
A/V Receiver to TV via HDMI.

Do the same for anything else you have. Everything should go through that receiver and you should just have the one HDMI to the TV. There are only a few exceptions to this rule and without knowing more about your system I cannot give more details.