Apple - 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/< username >/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).

They are stored as .aiff files in /System/Library/Sounds/.