A particularly striking example appears in Hawthorne's next romance, The House of the Seven Gables, in which Chapter 13 consists entirely of a metadiegetic narrative entitled "Alice Pyncheon," composed and read by the diegetic character Holgrave.
Source: wiktionary
The diegetic and metadiegetic aspects of the Joker's use of music also persist. Prince's "Trust" features diegetically in the festival parade as the Joker rides on his birthday-cake float, while the Straussian waltz reappears metadiegetically in the cue "Waltz," for the battle between Batman and the Joker's henchman as the Joker waltzes with a semiconscious Vicki in the background. This, and his comment, "shall we dance?" to Batman just before the cue starts again positions this as music that the Joker has control cover: it is more than simply heard in his head.
Source: wiktionary
Sound effects are often enlisted to externalize a characters' inner thoughts, nightmares, hallucinations, dreams, or wishes. We might hear, for example, the laughter of a child as a woman picks up a doll from childhood. This gives the scene a surreal feeling. This effect is often called meta-diegetic.
Source: wiktionary