I was playing Firfight and some semi-creepy music was playing when I started. About 1 or 2 minutes into the song (at this point I was hiding from some doom-grunts that got some lucky shots in) I thought I could here a voice. At this point all my skin started tingling and the hair on my neck rose. I called for my friend and I told him that there were voices.
When he came in he was all "I don't here anything." I said, "You just missed it." So I restart the film clip of the game, turn my TV WAY loud, go through it and...
Nothing. "Maybe you're going crazy." He said. "No way, I swear I heard them." Not even a split second pass and some crazy demonic sounding voice boomed out of my speakers. Me and my friend just sat there and look eerily at eachother.
This voice was definately creepy sounding. He said that it was probably backwards. As it turns out, someone else found it first and posted it on youtube... I think he called it the secret of windward or something like that. I found it on last exit, so that means its just some background music.
Oh, when played backwards it was a poem by... (can't remember) I guess there have been references to it in the other Halo games though...