Melodyne is a program that can be used to alter .wav files in order to change the pitch of its contents. The program breaks up the .wav file into areas of where it guesses words are. These areas can then be manipulated in many ways in order to make it sound as natural as possible, but make it sound correct and to the main tune. It is primarily used in the music industry to make pop music singers appear to be singing in the correct key. However, one person has used and still uses this program for making YTPMV's. HoZKiNZPooP uses this to pitch change with ease, and not go through the awkwardness of changing each pitch manually several times in order for it so sound right.
Melodyne "DNA" (Direct Note Access) is a program that is Melodyne, but it allows you to edit polyphonic audio. Polyphonic audio is basically what you get when you download music off of YouTube. This allows you to easily manipulate people's voices and for you to stretch them out without ANY type of echo.