Helium may be in short supply, but a little digital tweaking can still leave your favorite actors sounding like the Chipmunks. Take a clip from a film and either speed up the action or raise the pitch on the audio track for some high-pitched laughs.
Examples:
http://www.youtube.com/watch?v=ps0bzgVLSQk
http://www.youtube.com/watch?v=wWRSvJJ7F00
http://www.youtube.com/watch?v=HCXuTqpM0ZQ