The stirring “Ride of the Valykyries” opens Act III of Wagner’s opera Die Walküre. Eight of the nine Valkyries, the warrior daughters of Wotan, ride their horses onto the battlefield to gather up the dead heroes and take them to Valhalla, the home of the gods. They await their sister Brünnhilde, who arrives with Sieglinde on her horse.
Read a synopsis of the rest of the plot here:
The earliest-known use of the music in a film.
Some later examples.
An excerpt that begins as diegetic but becomes something else:
Wagner goes back to Vietnam:
Both diegetic and self-referential: “The Ride of the Valkyries” as a meta-narrative:
Ironic Wagner: In Fellini’s 1963 film 8 1/2, Marcello Mastroianni is a film director suffering from creative block. He visits a spa for treatment, where he is recognized by the other patients.
And then there’s this: