I think this is a bug in the book in that the music alias is not defined before it’s used as a qualifier. To do so, you either need to define a namespace music that is loadable or, more commonly alias music to some longer namespace (which is also loadable):
(create-ns 'domain.music)
(alias 'music 'domain.music)
In Clojure 1.11, the new :as-alias clause in require allows you to do this without loading any namespace:
(require '[music :as-alias music])