Another quick tip for you Appletosh users: Change Your Default OS X Sounds

It's easy:

1. Go to your Home folder (usually your name)
2. Go to the Library folder, then find the folder labeled Sounds.
3. Drop in .AIF files.
4. Next time you're asked by a program like Mail or iCal to select a sound for something, the list should be longer with your new sounds included in it.

Enjoy