Introduction to Non-Diegetic Sound

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. 

RESEARCH NON-DIEGETIC SOUNDTRACK
In common film music and soundtrack - a producer and director will hire a composer and sound producer to create sound based on information and research that the director/producer provides for inspiration and style. They will attach a storyboard and give clear structure to the sound and when its needed. It's important that if the sound is being created before the film is made that the story and style is very clear so that the wanted effect is created.

With our film we feel it's important that the non-diegetic sound doesn't add all the emotion to the film but that it's an aid. You should be able to watch the film and understand it the same way you would with non-diegetic sound.

Good examples of non-diegetic sound being used and natural sound being enhanced are in my studied short films such as White & Apricot, the sound is very much an aid rather than leading the film where as in Future Proof for example the sound is leading, more like a music video.