Hi @brieze ,
Couple of last suggestions.
1. Check the following setting: (realize that you may have done this already just covering all possibilities).
Go to Home > Settings > Sound > System >Default Notification Sound and ensure that it is set to a tone rather than 'none'
2. Backup your phone using the phone's backup feature, then reset your phone using the phone's factory reset feature.
Be aware that a factory reset (or restore- also known as a hard reset) will erase all your downloaded apps and User data. It will restore the phone to a factory default condition. It will be like when you first bought it. That is why it is important to do a backup first. Also ensure that the battery is fully charged before you perform a factory reset, that the charger is not left connected and that you have removed the external SD Card, (if any installed)
When the phone has been reset check to see if the SMS notification sound is there when you receive a text.
If it is, restore the phone with the backup you made earlier by using the phone's restore feature and when it has been restored , check to see if the SMS notification sound is still there when you receive an SMS.
If it is then the reset has fixed the problem.
If it is now faulty again a downloaded app must be causing the problem somehow. The trick is to find out which one. You will need to uninstall each of your downloaded apps one by one and test your SMS notification sound after each one has been uninstalled to find out which one is the cause of the problem.