Audio problem: To dispel Jennifer's suspicions that he's still in love with Gale, Dewey adds that Gale is cruel, etc. David Arquette's lips are not moving, though. Noticed also by the crew on the DVD commentary track. (00:39:40)
Audio problem: When Tyson is running down the stairs he yells something but his lips don't move. (01:19:55)
Audio problem: When the killer has on his mask and you hear "the voice," he does not need to use the changer to mask his voice. But we see that it is just a plain old Halloween mask.
Answer: Not expressly, though it could have been the date of some significant event (December of 1988) but someone as security-conscious as Sydney now is wouldn't be stupid enough to have her code be something that could be guessed.