I did something random but rather amusing this weekend:

1. Take your favorite video, it doesn't matter what.
2. Use Avidemux2, a great video editor to export just the audio as a WAV. To do this, select WAV PCM as your audio codec and then select Audio > Save from the menu.
3. Open the resulting .wav in Audacity and use the Change Pitch filter on the entire file.
4. TIP: -30% sound like low-pitch and +35% sounds more high-pitch.
5. Export your Audacity project as a .wav PCM file.
6. Now, return to Avidemux and select Audio > Main Track. Choose External PCM from the dropdown and now locate your saved WAV from Audacity.
7. Change the audio codec to FFmpg MP2, 128kbps.
8. Change the video codec to DVD(lavc) and make sure the Container is 'MPEG PS+AV'
9. Save your new video and laugh

It usually takes a while to encode, and the quality won't probably be great... But it's still pretty funny. Some of my mpeg's didn't work, for that I had to play to fiddle with the framerate & filters among other things for an hour or two before they turned out right.