I read somewhere that Bourne's husband who is a musician did the music cuts. But Hanyu himself also had some ideas about small things that should be placed in the music, like the very beginning (which is actually a recording of him drawing in breath) and also that whistling sound (if I remember correctly).I think it might vary between each choreographer and skater. I really liked this article with Shae-Lynn Bourne about doing choreography with Yuzuru. It isn't clear about who did the music cut however she did get a chance to research Japanese dance and incorporate movements from the movie before working with Yuzuru in person.