The Greek word “Diegesis” means a recounted story. Oxford’s English dictionary states “The narrative presented by a cinematographic film or literary work; the fictional time, place, characters, and events which constitute the universe of the narrative.”
In other worlds, the world of your film. And in your world there are sounds of two types:
Diegetic is the sound that is meant to be from the actual scene happening on screen like character voices, elements in the picture (cars, traffic, birds, etc) and music from instruments played on screen or from a radio or television (also known as “source music”). This diegetic sound can be on or off screen though it is always meant to be actual sound. Of course, nowadays in post-production, most of this is either created entirely by a sound designer and or sweetened by the mixer.
Non-Diegetic is the sound of artistic license. It’s the sounds of commentary, narration, sound effects and the music soundtrack. This is where many purists have claimed that the use of such non-diegetic sound is articifial and contrived. Danish filmmakers Lars Von Trier and Thomas Vinterburg’s Dogma 95 states “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). ” Of course Von Trier’s Dancer in the Dark breaks these rules. Dogma 95 may be more of a publicity stunt like nailing your manifesto to the church doors. There is no question that music can heighten the mood, project emotions and stay ringing in your head as your leave the darkened theater to return to the reel, I mean real world. How effectively you use the music determines the level of “realism” and whether you even remember there was music in the scene.