How is the default MP3 melody chosen on a Series 9 doorbell(RDWL917A)?

If you've associated your doorbell push to a downloaded tune and later delete that melody, the push will default to the next downloaded melody or to one of the mp3 doorbell standard melodies. If you have more than one doorbell push button, when you delete an mp3 melody that's linked to one doorbell push, it will also affect the other pushes in the system.