Was playing this in preparation for Halo 4, and noticed that in the second cutscene in The Maw (the one where Guilty Spark shuts off the self-destruct), he doesn't do his line about "a record of all our lost time! Human history, is it?". Does that happen for anyone else? Kinda weird, it's hard to imagine they deliberately took a line of dialogue out, but I also don't know how else that line would not be playing.

Hmmm is there a subtitle for it but no sound, or is the line just gone? Is it replaced by something?

I would guess it is a glitch, that game had a few small ones from what I can remember.