Try this... Issue solved!
I had this problem on my s8. Go to settings, apps, and find messaging. Look for "allow notifications" there. None of the other notification settings work if that one isn't activated. Hope this helps.
I'm having the same issue and my notifications are on. It should play my default notification sound like every other phone I have ever had but it doesn't.
I'm not sure if you've found your answer, but I thought I'd let you know of an option that can silence your text and app alerts. If you go to SETTINGS and then NOTIFICATIONS, you'll see where there's an option turn on ALL APPS. If you don't want every single app to be able to alert you, you can turn that option off and then turn on app notifications one by one.
If this doesn't help you, maybe it'll helps someone else.
Try, settings then "sounds and vibrations " then "volume" there you will have the volume controls for everything, I had the same issue, all was enabled but my notifications bar was turned all the way down. Hope it helps.
For some unknown reason Samsung decided to mute the default message app tone when the app is open in an active "conversation"
The notification will only sound off at the beginning. Once you open the message app and start texting, you will not get additional sound notifications unless you either close, or minimize the app.
So if you are carrying on a text conversation, and leave the app is open, you'll have to look at the phone to see when the other person replies. I hate that Samsung has done that. Thats why I use WhatsApp for most of my texting.