I have the same issue. I even replaced the built-in app notification sound with one of my own. It is still too quiet to hear. I have the same problem with the FB Lite Messenger app. I may create a volume-boosted notification sound (use an audio editor to increase the audio volume of the sound file and save as a new file) and see if that helps. If have Facebook and FB Lite Messenger in my Secure Folder and wondered if that was the problem.
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