I don't remember much about case system theoretically-wise, so please correct me if I got anything wrong.

"Ты" is the only word in Nominative here, because it's a subject (a subject is always in Nom.) "Музыку" is accusative because it's a direct object, i.e. an object of the action "to love". "Какую" refers tо "музыка", and thus it adopts accusative too.

Maybe it will be easier to see if you mentally change the word order and omit "какую": Ты любишь музыку? Omitting "какую" changes the meaning, of course, but the inner mechanics of cases is the same.