Sound- Non diegetic

Sound whose source is neither visible on the screen nor has been implied to be present in the action: Narrator’s commentary,
Sound effects which is added for the dramatic effect
Mood music
Non-diegetic sound is represented as coming from the source outside story space. The distinction between diegetic or non-diegetic sound depends on our understanding of the conventions of film viewing and listening. We know of that certain sounds are represented as coming from the story world, while others are represented as coming from outside the space of the story events. A play with diegetic and non-diegetic conventions can be used to create ambiguity (horror), or to surprise the audience (comedy). Another term for non-diegetic sound is commentary sound.

This is sound which does not exist within the story of the film, but is put onto the film in post production. This type of sound could be in the form of a music soundtrack, voice over or extra sounds which enhance the meaning of elements within the film.

For example Bernard Herrman's haunting soundtrack for Martin Scorcesse's "Taxi Driver”, would become under the definition of non-diegetic sound. The shrill and repetitive violin sounds which Herrman created as the soundtrack for the shower scene in Hitchcock's "Psycho", function as a musical echo for the knife stabs inflicted on the character of Marion Crane.