Nota: Eu não vi os filmes mencionados na pergunta.
Quando a música aparece em um filme apenas porque aparece no universo, ela é chamada de música diegética . A razão é artística, claro. Na maior parte do tempo, é usado para contrastar filmes super construídos que, do ponto de vista de artistas que usam isso, não permitiam boas histórias ou levavam a filmes caros, o que previne certas histórias contadas por diretores que não podiam pagar faça o filme. Um famoso manifesto que emprega isso é Dogme 95 . Eu apenas cito a parte importante da página da Wikipedia para os objetivos e (outros) métodos:
Goals and rules
The goal of the Dogme collective is to purify filmmaking by refusing expensive and spectacular special effects, post-production modifications and other technical gimmicks. The filmmakers concentrate on the story and the actors' performances. They believe this approach may better engage the audience, as they are not alienated or distracted by overproduction. To this end, Lars von Trier and Thomas Vinterberg produced ten rules to which any Dogme film must conform. These rules, referred to as the "Vow of Chastity," are as follows:
- Shooting must be done on location. Props and sets must not be brought in (if a particular prop is necessary for the story, a location must be chosen where this prop is to be found).
- The sound must never be produced apart from the images or vice versa. (Music must not be used unless it occurs where the scene is being shot.)
- The camera must be hand-held. Any movement or immobility attainable in the hand is permitted.
- The film must be in colour. Special lighting is not acceptable. (If there is too little light for exposure the scene must be cut or a single lamp be attached to the camera).
- Optical work and filters are forbidden.
- The film must not contain superficial action. (Murders, weapons, etc. must not occur.)
- Temporal and geographical alienation are forbidden. (That is to say that the film takes place here and now).
- Genre movies are not acceptable.
- The film format must be Academy 35 mm.
- The director must not be credited.
Quase nenhum filme segue completamente esses princípios.