Apparently I can't link to my external content, so here is my insight:
After extensive investigation and testing using two different Lumia 950s running the latest version of Windows phone 10 (OS build 10.0.10586.218) I can provide the following insight:
? Ring tones must reside in the Ringtones folder (and it seems that this folder must remain on the phone not on the SD card though I haven?t tested this extensively)
? MP3 and WMA files are supported. WAV do not seem to be usable. I haven?t tested anything else.
? The one CRITICAL requirement is that you MUST copy to the Ringtones folder from your PC over USB. Every other approach I?ve tried, including Bluetooth, download from OneDrive, copy from SD card using the Files app, etc. has failed.
? The following make NO difference whatsoever to whether the file will or won?t appear in the ring tone selection drop-down (personalisation, sounds, ringtone)
○ Genre (ignore any advice about this needing to be set to a genre of ?ringtone?)
○ Sample length (ignore any advice about this needing to be set to 40 seconds or less)
○ Bit rate (I had this running with bit rates from hundred and 128 to 320 kbps)
○ Metadata (it seems to largely ignore all the metadata in the file header, though there is something slightly odd going on and it seems to extract the track name correctly even if it has a leading track number in either the file name all the track name)
? You cannot create folders within the Ringtones folder and use these to organise ring tones and alerts; anything within a folder is ignored.
Once you have added ringtones in this way in then select the ring tone from Personalisation, Sounds and the Ringtone drop-down; your custom sounds will be at the bottom of the list and there is no way to remove the undesirable built-in tones.
Custom notifications for texts etc.
Exactly the same rules apply for notifications, except that they are added by going into Manage App Sounds at the bottom of the Personalisation, Sounds screen. You then click on each app that provides notifications and select your notification sound. Since there is no way to tell the length of the sound you?re selecting, you may wish to change the notification sound names to something like ?alert-soundname?
The Alarms App bug
My final observation is that the Alarms App ignores the sound you select through the application. However my big discovery is that you can set the alarm sound exactly the same way that you set notification sounds (see the previous paragraph). Clearly this is a bug, but it?s good to have a workaround.