For example, the description of fictional music in books such as Doctor Faustus by Thomas Mann or A Clockwork Orange by Anthony Burgess has been described in such a fashion.
Similarly, the "fictitious music" of the fictional composer Van den Budenmayer can be heard in movies Blue and Red in the same trilogy and in another film by Kieślowski, The Double Life of Veronica.
Similar examples include Hubert Bath's Cornish Rhapsody from Love Story, Nigel Hess's Fantasy for Violin and Orchestra from Ladies in Lavender, and Max Steiner's Magic Isle Symphony from City for Conquest.
Some film compositions are based on literary works, such as the concert segment in The World of Henry Orient, actually composed by Ken Lauber.
That piece is based on writer Nora Johnson's description of the concert in her novel The World of Henry Orient [Wikidata], that inspired the later film.
Hypothetical musical instruments using live animals, intended as thought experiments or as bizarre humour, include the cat organ and the piganino.