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 a 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.
