I remember trying to accomplish this when I was a kid, only I didn't have the patience to slow the record down with my finger. We did have some cassette recorders, though, and I discovered that if you held down the pause button halfway on one as it was rolling, the mechanism used to pause the cassette would actually speed the tape heads up a little bit. This meant you could speed up the playback and make your own wacky Chipmunk songs if you want (hint: make them sing something dirty for extra laffs) -- or, if you held the pause halfway down during recording, you could make SLOOOOW GIAAAAANT VOIIIIIICE. I remember trying to slow down a recording of the Chipmunks by playing it into the fiddled-with tape recorder, but I don't remember results as, well, downright creepy as this.
So who did this particular monstrosity, anyway?