Somebody has to do that sound file I guess.

I remember me playing around with some friends talking backwards, recording it and then using the simple Windows recorder or something to then replay it backwards again.
The aim was to make it sound normal then, so your backwards talk had to be good and precise.

Guess what, it was also very much fun to train such stupid things for some time and then go out talking like that all the time since most of the fellows involved established some very good skills there in hearing and speaking.