Where are the Mac OS X alert sound files located?
Any files located in the following locations will available to choose as alert sounds:

/System/Library/Sounds/
/Library/Sounds/
/Users/&lt; username &gt;/Library/Sounds/


(Note: By default the latter 2 folders may not exist, but if created and populated with files, you'll be able to choose them in the Sounds preference pane).