The Sounds Of Star Wars

Diegetic sound- Sound whose source is visible on the screen or whose source is implied to be present by the action of the film:

voices of characters
sounds made by objects in the story
music represented as coming from instruments in the story space ( = source music)
Diegetic sound is any sound presented as originated from source within the film's world
Diegetic sound can be either on screen or off screen depending on whatever its source is within the frame or outside the frame.

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).

Star Wars: The Force Awakens Trailer-
Examples of non-diegetic sound:
There is a clear definition in the instrumental score that plays from the beginning that shows the audience who the main Protagonists and Antagonists of the film are. In the first quarter of the trailer are whilst introducing the heroes this non-diegetic sound is softer and non-threatening, however, when the villain is introduced the music becomes louder, more instruments are heard and the score starts to be played in a minor key. Later on in the trailer the score changes to become two recognisable pieces of music from the original trilogy. These two scores represent two main characters that to a fan of the Star Wars saga would be instantly recognisable. The non-diegetic sound in this trailer is used to initially introduce the audience to three new characters and establish whether they are good or bad characters. Later on the non-diegetic sound is used to link these new themes to already existing characters within the story so far. This allows the film-makers to establish new characters and reinforce old ones using non-diegetic sound alone.

Diegetic sound:
49 seconds into the trailer an ominous voice over is heard. The deep and distorted voice is a clear indicator that this is a villain we are hearing before we even see what he looks like. In this case diegetic sound twined with the non-diegetic gives the audience a clear idea of where this characters loyalties lie. This character is saying "I will finish what you started" unlike the non-diegetic sound this needs visual aid for it to make sense ie. a close up of Darth Vader's helmet. After the antagonist is introduced the diegetic sound becomes a combination of dialogue and sound effects. These sound effects tell the audience that this will be a sci-fi film but also make the trailer recognisable to fans of the series. For example, spaceships such as the X-Wing and TIE Fighters are included and are from the original films, as are the instantaneously recognisable sounds of a lightsaber..