I have a ringtone and that works, application notifications work, but any messaging notifications don't work and no system sounds work (camera, charging, etc)
I was told the last update was making it so after market apps don't work anymore (like apple does) but I've deleted anything to do with sounds and it didn't fix the issue, beside the fact that they have nothing to do with system sounds.
When I called Samsung they told me they were aware of the issue and a new update was coming out to fix it, they didn't have a timeframe so I was wondering if there had been any updates on when this is coming.
I had an issue like this before with my S7 and the Oreo update. My only solution was to factory reset. Seemed that there was a bug where my audio file wasn't being properly detected and wasn't giving actual alert tones. Once I reset my device, the notification sounds started working again.
If you choose to do this method, ensure you do a backup of your device with SmartSwitch.