Demo consists of 2 melodies : The main melody (white) and the auxiliary melody (gray). The main melody is generated from the lyrics and the auxiliary melody is generated from the chords of the song.
Our music box is an 18note-type music box.The notes will be reserved for the main melody first and the rest will be filled by the auxiliary melody. For instance, If your chosen song contains 13 notes of the main melody , it will leave 5 notes for the auxiliary melody. More auxiliary melody in a music box will enhance its quality. ( Read more detail in The Limitations of Music Box )
The song with many repetitive notes is not recommended. It will give you fewer notes for an auxiliary melody.
When the song plays until the end of section, the section will start a new loop. So that the song sounds seamless
Chorus section is the most preferable section to make a music box.The whole chorus section will give you the most seamless looping.