@Promisefrancis: If you head to Settings > Apps > Select an app > Notifications > Sound, then you may find that it is has 'App approved sound' as it's default. If you see this, please refrain from changing it as, as far as I'm aware, it can't be changed back via the list of options. You may wish to try heading to Settings > Apps > Tap the 3 dots in the top right > Reset app preferences to see if you can restore them in this manner, however please be advised that this will remove any personal settings you have applied to all your apps, including any sign in details.
If you used Smart Switch to transfer the data from the old phone this is the main reason. Try re-installing the apps in order to get the default Notification sounds