I saw this film via a past recording a week or so back, and while I can't recall specifics.. I do remember thinking the sound mixing within the actual show could well be as you describe. When the TV is free I'll try the VOD version again and see if I get the same.
Edit: Just tried it via VOD in SD & HD (the Sky Cinema "Most Popular" section only shortcutted to SD, I had to browse round to find HD, which is odd but another issue) - yes, I think it's just the movie soundtrack, dialogue is just quiet.