From http://blog.airdrop.org/archives/000014.html -
"So, the story goes that co-worker A got sick of co-worker B's constant profanity whilst playing Counter Strike. Co-worker A then hid a microphone next to co-worker B's desk, and captured this stream of rantings. Then, as co-worker A was a member of an electronic music-making concern (http://www.ingsoc.org), he endeavored to edit it all together. The end result was this: The Terrible Mr. G."
http://overstated.net/media/terrible_mr_g.mp3
Although this was made off of someone playing Counterstrike, the entire concept is pretty much universal among FPS's...including Marathon. Hope you enjoy it :)