Try clearing Data, cache and force close the message app.
'll leave a Link to register so you can communicate here as a guest account you can only post questions but can't reply. https://forums.androidcentral.com/showthread.php?t=409154
Welcome to Android Central! Do other apps give you audible notifications? Go to Settings>Apps, select the messaging app, then Notifications -- what's selected there? Can you change the notification tone to something else?