Sound (Diegetic/Non Diegetic)

Diegetic sound is sound which exists within the world/universe of the film. Often meaning it has a visible source in the frame but not always, It is also common for the characters to be able to hear the diegetic sound. Examples of this may be music at a party which the characters are dancing too:

Non-diegetic sound is often sound design which the audience can hear, without it existing in the world of the characters. It is unlikely to have a source in the frame unless it transitions from diegetic sound to non-diegetic sound. For example, the Russian classical piece being used as a non-diegetic soundtrack as the characters travel within the scene: