Film Language - Sound

  Sound in film can take two forms: diegetic or non-diegetic.
  Diegetic sound is sound which the characters in the story can hear, as well as the audience. This will usually consist of things such as conversation, and the other noises which come about due to the action of the characters. There may also be music playing in the background which the characters can hear as well as the audience (e.g. if they are listening to music on the radio).
  Non diegetic sound is sound that can only be heard by the audience (the characters in the world of the film cannot hear it). This usually involves things such as background music used to set the atmosphere. To a certain extent, it also includes the voice of a narrator (if there is one).