Varispeed (changing the speed and pitch of audio like you could do with tape) is one of my favourite audio effects. It was more common in the analogue days because it was easier to slow tapes down that to do the equivalent with digital. [Cockos’ Reaper has a great varispeed facility but several DAWs (Sonar, I’m looking at you…) have yet to implement anything like it. This is actually one of the main reasons I’ve started to migrate from Sonar to Reaper.] Varispeed is the thing that lets you get the “Chipmunk” vocals.
But sounding like The Chipmunks isn’t why it’s interesting – that’s even to be avoided! Nor am I talking about slowing tape down to half speed, playing and guitar solo and pretending you’re Eddie Van Halen. That’s one possible use but is rather crude. No, it’s about getting new and interesting sounds out of the same instruments.