I found some old MP3s in my music folder of Halo 3 soundtrack material in a rough yet interesting state. They're full of atmospheric sound effects -- jungle bird calls and the patter of rain in a Sierra 117 track, or desert wind blowing through Tsavo Highway -- but they sound deliberately placed, not like random in-game noise. Now that I think of it, I vaguely remember a few tracks leaking before the soundtrack officially launched, but I can't find anything mentioning such a leak anywhere online.

Am I imagining this? Where did these bootleg tracks come from?

Kilimanjaro
On the Prowl
Unwelcome Visitor
Full Contact Safari
Into the Storm

(Not sure if I got the titles from elsewhere or named them myself, fwiw.)