Share your experience!
Let me try to understand, whenever you change the notification sound for the messaging app it doesn't change at all?
What about clearing data for stock message app,followed by a force restart?(power button+vol up until the phone vibrates).Do backup your messages before.
How do you set the messaging notifiaction?
messaging app > menu > settings > notification sound?
Here are some suggestions
Alternatives on How to backup Xperias