I know it's a very specific question, but does anyone remember?

I don't remember hearing it...I don't even remember hearing it in Halo 3 (sorry if it obviously is)

Correct me if I am wrong, but I think it is at the part when you choose whether to kick the guy off the building.

I found a list of all the game that have the Wilhelm Scream

http://kezins.com/2008/06/weve-all-heard-it-the-wilhelm-scream-video-games/1736

"Correct me if I am wrong, but I think it is at the part when you choose whether to kick the guy off the building. "

the wilhelm scream actually is in gta 4  there is a scream that sounds like it but the actual scream itself happens only once in the whole game its on a mission called the "final interview" where you kill goldberg in his office,once youve killed him and got the files go outside the office there should be more gaurds (kill them) and once you go down the first staircase at the top of the other one there is another guard shoot him in the leg once (with a pistol) he should dive down the staircase then he will do the wilhelm scream. very funny. sometimes he does'nt fall or the scream doesent happen be patient keep trying and he will eventually do it.

It also happens in the mission near the beginning where you have to chase down Albanian loan sharks and one of them has a knife and you fight in a building being constructed. If you push him back into the glass window, he'll fall and trigger a cutscene and do the scream. This doesn't happen if you kill the dude before he's close enough to the window.