You experience is the exact opposite of mine. Default sounds (those set with the Windows installation) work fine, as do MP3 files that I have customized for use as ring tones.mp3 never accepted on 950XL. Drag drop of mp3 file convert it to .wma that is playable on 950XL. All my sounds on 950XL are .wma format.
That file list does not show the system ringtones that come with Windows 10 Mobile. I don't know where they are located, but they are either hidden files (probable so that files are not deleted by the user) or stored somewhere else.In my 950XL Ringtons folder no audio file other than .wma shows at all in the list of ringtons.