In the song, Into the Babel, some chanting can be heard, slowly getting louder and changing pitch till it climaxes. But I've heard the chanting in other songs as well, including 'Trigger Situation' from Dirge of Cerberus (the song heard in the pre-game movie). I'm wondering where I can find that chant.
The 3rd Birthday - Into the Babel http://www.youtube.com/watch?v=5dkxeYc1smA
Dirge of Cerberus: Final Fantasy VII - Trigger Situation http://www.youtube.com/watch?v=fjzcNsJVol0