Add whatever audio you want to the "Notifications" folder, in the internal storage.
That's it.
Video there if you need it, i copied a song from my ringtones folder just as an example, and added it into the Notifications folder, and that made it available in notification settings.
glad you've managed to follow my instructions and made a video to help everyone. at least now everyone can enjoy the ring tones they want and not the ones that are preset into the phone. took me a few months to figure it out on my s20.