Hi,
As dialabrain alludes to they need to be in the two folders rather then where you have put them.
I have tried putting an Alias of the ~/Library/Sounds folder in one of the two locations but it does not seem to work.
However that might be due to not having the "right" format sound files in my Sounds folder.
Once you have them in the the correct location the .plists in those locations also detail what files are in there.
This may mean that you need to edit the .plists to list the new sounds as well as having them in the list.
It seems that Messages, in the drop down leroydouglas has shown reads the .plists rather then gathering the info as iChat used to do.
This does seem to mean that the iPhone can use any sound, even part of a Tune, to be the Ringtone or Alerttone where as it seems the Mac can't despite GarageBand being able to create Ring Tones.

8:45 pm Saturday; September 8, 2018
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
MacBookPro 15" 2016 (High Sierra 10.13.x)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)