When we write stories authors often have theme songs for books or characters. Certain songs remind you of places or moments of characters.
Some writers use music to motivate them. I did this alot for Mr Right and Other Mongrels but not so much for Hearts Afire. I think maybe because Allegra seemed like the sort of girl (like me) who might have her own theme song and Cassie seemed a bit too sensible for that.
Still there were musical moments. Momemts you could play out like a nice video clip.
For example, if you read Hearts Afire this song might be a good one for Cassie at the start of the book.