Diegetic

[Couch Potatoe]

A sound which can be [paired] with what is [happening] in a film or on TV, and is either visible, or is [relevant] to the action.
Non-diegetic means a sound that cannot be seen on screen, nor has any direct relevance.

[Tracie Lynn]

Diegetic music in a film or TV programme is part of the action and can be heard by the characters, rather than being just for the people watching to hear:

[Sonia]

(of sound in a movie, television program, etc.) occurring within the context of the story and able to be heard by the characters.