Addition.
I have used GarageBand for cutting down longer items to make ringtones.
Flip4Mac allows you to play some sounds not normally played by Quicktime.
Quicktime itself has several Export options that might work.
History.
In the past iChat and versions of Messages up to an including Messages 8 in Mavericks listed some Sounds from Inside the App (In Application right click the app icon and then Show Package Contents and then navigate to resources)
Also in the drop down the change the sound were the System Alerts sounds.
Messages 7 and earlier also saw the ~/Library/Sounds folder.
Yosemite, which still calls the app Version 8, despite the significant changes, has totally changed the Sounds used in the app.
The ones within the app still, apply to the sound given to all the other actions, Buddy's coming On and going Off Line in Buddy Lists, the 'swoosh" like sound for Sending Messages and the like.
Now.
It does seem strange that Apple have two Sounds in different places (app and the Folder in the pic) in different formats that sound the same and essentially the same name.
Having said that neither appear in the list of sounds that can be played by the app for this particular function.
8:30 pm Monday; December 8, 2014
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad