Where are the alert tone audio files located in OS X? In what directory on OS X are the sound files located for the alert tones in Messages and in FaceTime? I'm not talking about the alert sounds in System Preferences; I mean the ones that are also on iOS—like Aurora, Bamboo, Chord, etc.—in the list in this screenshot:
Alerts are located at
/System/Library/PrivateFrameworks/ToneLibrary.framework/Versions/A/Resources/AlertTones/
Ringtones in
/System/Library/PrivateFrameworks/ToneLibrary.framework/Versions/A/Resources/Ringtones/ 

If you're looking to add your own - you can't.... or rather you can but they will be ignored.

I've found the best method to add my own is to replace one of the existing sounds with one of my own, renamed to match.
See https://apple.stackexchange.com/a/169907/85275

For completeness...
System alerts - Sosumi etc - are in /System/Library/Sounds/
User sounds are in ~/Library/Sounds/