Hopefully you have found the solution, but thought I would share what worked for me in case it would help someone else! I was having the same problem (J7), and I had read through lots of message threads and watched YouTube videos with no luck. One of the replies on this thread pointed me in the right direction... I clicked on the 3 little dots at the top right of my messaging screen>Settings>Notifications>Notification sound, and on that screen I realized that the slider for Notifications Sound was turned down... turned it back up, and now it's working. No clue how that had gotten turned down, guess I will have to keep an eye on that from now on.
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