Ringtones, notification sounds and alarms are stored as sound files in separate subdirectories in /system/media/audio.
If you enable root access in your FP2, you can use a file manager that can use root privileges to navigate to those directories and copy/paste any sound files in them you like, and they’ll be there until you update to a new version of Android.
After doing the above, when you go to set your ringtone in the settings, your custom sounds will pop up in the selection menu - no special apps required.
You need neither a special app nor root to use custom sounds. Just copy the audio files to the “Ringtones” or “Notifications” folder. They are at the same place where the “Download”, “DCIM” etc. folders are. If they’re not there just create them.
Audio files placed there then show up in the ringtone / notification settings.
I’ve just tried out on 7.1.2 (FPOS 18.09.2) and an mp3 file stored at /sorage/emulated/0/Ringtones was found/could be selected and used in stock phone app (and it has also survived a reboot).
One possible explanation for this manifestation of ‘Your Mileage May Vary’ is that if you have a lot of media files, it might take a while (depending on use / battery charge) for them all to be indexed (and show up). This is also why battery life can be atrocious for a couple of days after an update that affects the media storage.