I don't have a manual for the Sky box but if it's putting audio on optical, the hdmi audio is usually missing. Try looking around the Sky box for options of where it sends audio as well as trying stereo, 5.1 dts, or others.
Just had a problem with my Onkyo receiver giving me green tint problems over HDMI. So I hooked my Sky "cable" box straight to my Samsung 8000 tv on HDMI1. Perfect image, but no sound. If I switch back to receiver image and sound immediately works (though still with green tint).
Switching cables doesn't work. Tried it on HDMI1 and 2. Put volume up and down and mute, on both the box and the TV. No luck.
How can it work over the receiver but not directly?? How can it be wrong?
ps.: I'm bypassing this problem using the optic cable for sound, but that's not nice!