Since Big Sur, this is no longer a valid method. The sealed system volume will not let you change the files.
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/
/System/Library/PrivateFrameworks/ToneLibrary.framework/Versions/A/Resources/AlertTones
and I'll accept it. :) – jsejcksn Oct 09 '15 at 09:59