Hi
I have the same order of play: 1. Bluray player - sound through HDMI; TV - sound through digital optical lead and last Sky HD - sound through digital optical lead.
I wouldn't say that the Sky was overly low volumed just obviusly lower than the rest. Have you tried a different digital cable??
Samsung PS-50C96
Marantz SR6003
Chord Active & 2x Supershield
2x Copper-Line Alpha
Sony BDP S360
Sky HD
True Colours Industries Coral
Arcam Apha 7 CD
Van den Hul D - 102 III
Quad L11 Fronts, Rears & Surrounds
Carnival SilverScreen
REL Strata