Which of the following is an example of 'non-diegetic sound'?

Non-diegetic sound refers to audio elements that do not originate from the world of the film or scene itself; rather, they come from outside that world and are not heard by the characters. Background music during a scene fits this definition perfectly because it enhances the emotional atmosphere and storytelling but is not perceived by the characters within the narrative.

In contrast, dialogue between characters and sound effects are diegetic, meaning the characters can hear them and they play a direct role in the story. Similarly, ambient noise represents the sounds that exist in a particular location, contributing to the realism of the scene and also being part of the characters' experiential reality. Thus, the distinction of non-diegetic sound primarily lies in its function as an external musical or auditory element that enriches the overall immersion for the audience without affecting the characters directly.