The "Samsung Gear" app was recently renamed to "Galaxy Wearable" so I'm not sure which one you have on your S9+. Inside that app:
1) Ensure that "Settings" is selected at the top and click on "Notifications"
2) Ensure that the top switch is set to "On"
3) Click on "Manage Notifications"
4) Ensure that the app that you want notifications from has it enabled.
Now leave that app and open the phone settings.
1) Click on "Notifications"
2) Find the app that you want notifications from and click on it.
3) Ensure that the type of notification that you want is enabled and it has a setting which includes "sound".
On your watch, open the settings.
1) Go to "Sounds and Vibrations"
2) Ensure that "Sound mode" is NOT mute. Mine is set to "vibrate".
Also, FYI there are dedicated Gear forums like the S3 forum at this link
https://forums.androidcentral.com/samsung-gear-s3/