Do any of your notifications have sound? I have often seen the default setting, in the system settings, get set to silent. Usually through an accident, some times because of a system/app update or after uninstalling an app that installs ringtones. They sometimes take the sounds with them or disable any that were downloaded. The equivalent of taking their ball and going home.
If your other notifications work and the system isn't set to silent, I'd try the stock Samsung app. It only takes a second to switch, test it and see if you're getting sound for any SMS.
The way your post reads none of your sounds work "in" GM but is that when you attempt to set one or do you mean for any contact sending a message? Which brings up is this for one specific contact or all contacts/SMS? If just for one or two be sure there's a sound set for that contact. In side of the conversation use the 3 dots to view details and be sure there is a sound set and that it isn't set to silent there.
You also say that you did a force stop "to clear the cache". Force stopping may not clear the cache. Go to the app page>Storage>Clear cache just to be sure and certain.
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