Gerald McBoing-Boing's Symphony (1953)

Gerald McBoing Boing, a little boy who can't talk but can imitate any sound, is working as a one-man sound effects department for a radio station. When a scheduled symphony orchestra does not show up, Gerald replaces them. He is doing fine until he mixes up the score with the sound effects script, and creates an original symphony. He is fired, but the critics hail his work as that of a genius.