Where are the alert tone audio files located in OS X?

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/