Do you have GarageBand?
Here you have the process!
My GarageBand opens my iCloud Drive. In your case it may not be so. Nevertheless the process remains the same. This helps because accidentally if you open up the iCloud Drive using GarageBand you can get back to GarageBand.
Music bust be on your iPhone not on iCloud Library.
When you select the song Hold and tap the Song delicately for it to populate it in Audio Recorder.
Watch the GIF below to convert any music to RingTone
This is made on iOS 13.3.1 iPhone SE
