Diegetic and non-diegetic sound is all about whether sounds from video or film are heard by the audience only or characters and audience at the same time. Film music sounds are sounds that characters are not able to hear, the reason being is because film music is just an effect to enhance some kind of mood for the audience. This is non-diegetic sound. Therefore, diegetic sounds are sounds characters can hear. For example, sounds from a nearby construction site that exists in the film, or a radio playing music. The movie scene below taken directly from Tom Hanks’ Forrest Gump is an example of both diegetic and non-diegetic sound.
All sounds that exist within the film such as Jenny’s scream, the rattling bikes, the dramatized sounds of Forrest’s leggings breaking are all diegetic sounds since the film’s characters can hear them. Though Forrest’s narration over his child self and film music are all non-diegetic sound, something only the audience hears.