Hello, welcome to the Samsung Community and thank you for your feedbacks. I understand your concerns with your messaging notification sound and I will be more than happy to take a look into this with you.
If your messaging app window is still open after sending a text message, when you receive a reply you will not make a notification sound.
To receive message notifications while still in the app, you would need to enable in app feature.
Open your messaging app
Tap Settings
Tap Notifications
Tap In-app notification settings (In notification settings you can also adjust any notification settings to fit your liking)
Here is a link to some additional information on App Notifications
