I'm missing a cutscene for the Brain Tumbler level for my Xbox-version game.
Well, it's probably something you didn't trigger. There are a bunch of cutscenes that have to be triggered at the right time or you'll never see them.
Yes. An easy one to miss is the one that plays if you go back into the isolation chamber grounds after doing your first bit of business with Sasha, rather than just continuing. Can't remember if that one falls under brain tumbler or campgrounds, though!
Is that the one with Lily? I think I found that with a BINK veiwer, but can't remember seeing it ingame.