If you are sure you have "Notifications" turned on but are still not hearing any notification sounds, then also make sure the volume level for notifications is "turned up".... This was the problem I saw on my own S7... Turning up the volume slider for notifications fixed my issue. Perhaps it will fix yours too.
Do these steps: 1. Open up settings, do NOT select "Notifications" (the obvious choice) but rather select "Sounds and Vibration".
2. From there, select "Volume".
3. "Make sure the "Notifications" slider is all the way to the right. .... Done!
I have no idea how this slider got pushed to the left on my phone.... but there it was... Moving it to the right fixed the problem for me.