@Richardr1 - Thanks for the link. I also note that the AVforums thread links to a Sky forum thread. The some of the later posts in the Sky thread are describing precisely the same issue that I have been experiencing. This makes it very clear that this is a Sky issue, rather than something specific to Sky via Virgin Media.
Last night I was watching a film broadcast on Sky Cinema Premier. It did not take long before the film came to a couple of scenes that suffered from going very quiet during sequences where the sound should be louder than average. I stopped watching, and a quick check indicated that the same film was available on Sky's on demand streaming service. I tried this version, and the sound was pretty much exactly as it should be, an order of magnitude better than the broadcast version.
This narrows things down further, the issue relates to broadcast Sky Cinema only, and not Sky's on demand service, which seems fine.
I honestly do not think that the reason that broadcast Sky Cinema sounds different to due to rules relating to sound compression. Maybe there are such rules, but the sound is so bad, so plain wrong, that I think it is a case that whatever compression Sky are using, they are using it incorrectly.
So clearly this is an issue for Sky to fix. I guess that all we can ask of Virgin Media (who I presume pay substantial amounts of cash to Sky) is that they highlight the issue to Sky, state that they have some grumpy customers, and politely request that Sky sort out their technical issues.
