What is the term for that style device when a soundtrack turns out to be heard within the reality of the film?

Question

I believe the initial scene of Eyes Wide Shut is an example for this. I am sure I've heard a fancy term for the use of a piece of soundtrack that turns out to be heard within the film itself. How is that called?

Answer 1

You may be referring to diegetic sound, which is sound whose source is established in the scene: a piano in the background, a car revving up, a person speaking -- all seen in the shots (or clearly present, even if off-screen).

However, non-diegetic sound is sound that is not actually present in the scene: background music, an exaggerated noise from an unspecified source, the voice of a person who is not present.

These terms are mainly used in academic writing -- almost never on an actual film set.