Either sample packs, samples they, themselves, collected, or something similar. Just google "vocal sample pack" and that will give you an idea. After that, they can edit it into all sorts of things. I mean, you can sing, monotone, into a mic, then autotune, pitchshift, and all kinds of other things to turn it into something completely unrecognizable.
Alright I can not get over how good the video editing is. I NEED the file to put in on my wallpaper engine as a background. Combined with the breakcore it truly feels like I am ascending.