Change alert sounds on messenger for mac

I just downloaded the new version of Messenger for Mac that recently came out. I was hoping that you would be able to change the alert sounds like when you get a new email but the really didn't seem to change a whole lot. New interface, being able to change peoples names on your list and searching for names and emails. Anyways, if anyone knows how to change the alert sounds it would be appreciated. I looked in the Resources folder in the Package Contents but couldn't find the audio file that it uses for default.