Android 4.4.2 FB Messenger Notification Sounds - where are they stored?

[AC Question]

I'd like to replace the notification sound for FB Messenger "Message received" with my own custom sound but as far as I can make out there's nowhere on my phone where the stock sounds are stored. Can I do it and if so how?

My phone is not rooted and I cannot root it cos I use Barclay's mobile banking app and it's used far too often for me to do without!