eh? Sin appears, you defeat its spawn, you get sucked up, you find yourself floating in an arena looking place (that's really the place you (SPOILER)defeat Jecht at the end) inside sin, you then find yourself in Baaj.

Is the bit you speak of inside that range? If so, I'm pretty sure there's vocals when there's supposed to be vocals. You might just be seeing some dodgy lipsyncing from when they dubbed the english vocals.