Well yes, they are modified with the computer.
Here is a step by step tutorial about how to reproduce a chipmunk's voice or Fred's.
- Get a song
- Download audacity for free here: http://audacity.sourceforge.net/download/ (PC Linux and Mac version available)
- Import the song to audacity, you can drag and drop it or go to File, Open…

- Click on Edit, Select, All (Ctrl + A)

- Click on Effect, Change Pitch…

- Now here's the fun part, Select the settings same as the picture below.

- Click on OK
And here we go, a Chipmunk song!
To convert the chipmunk file into .mp3, click HERE.