I was thinking so much about Narnia when I wrote that! I even went so far as to wonder whether Witch Week was the Silver Chair of the Chrestomanci series, lol.

Caprona worked better for me than Witch Week, I think not because of the singing because that's a pretty minor aspect of the story (and it's SO hard to write well about music, it's like dancing about architecture as the old saying goes), but because of the Shakespearean overtones of the setting, and because Tonino's family is so great - I think it's the only non-dysfunctional birth family in any of the Chrestomanci books.

Did you ever read Seraphina by Rachel Hartman? That's my favorite fantasy book about music.