The correct answer was in fact given in this thread, post #9 I think… not sure why some people didn't see it. In any case, for future reference :
If you want your custom ringtone and notification sounds to appear in the default list your device shows you when you go to …
Settings > Sound > Ringtones -and-
Settings > Sound > Notifications
.. you just need to put them in the following folders :
/Media/Ringtones
/Media/Notifications
You'll usually see these folders referred to in all lowercase letters, for some reason, but they work just the same using first-letter caps as above, if you tend to be anal about that sort of thing.
Note that the names that will show on the lists will be the audio files' "real" names, as defined in their file properties (tags), meaning you'll only see the actual filename of an audio file appear on the lists if the file in question doesn't have a different name defined in its properties. So if you decide to name your current default ringtone something like "Ringtones – Default – May 2015.m4a" but that file's properties says its title is "Zedge.net-- 901417" for example, don't look for it under 'R' in the list of ringtone choices your device shows you, because it'll be under 'Z'.
Btw there are free tools to change audio files' data properties (tags) if you want… there's one for Windows I've kept around for years, lets you view the tags of every music file all at once like on a big spreadsheet, and allows you to do stuff like sort them by column ( i.e. by property) and bulk-modify a bunch of them at once. Don't know if it's still available but software aggregators like cnet probably still archive it. Called Mp3tag, just Google it.