Music that the characters can hear. It is important to the plot as it is actually a part of the story.
Extra-Diegetic
Music that the characters can’t hear. Useful for setting the scene and adding atmosphere, but not important for the plot itself. Most film music is extra-diegetic.
"Mickey mousing"
Composers often need to create music that closely mirrors the action. This is often known as “mickeymousing” but composers now tend to do it in more subtle ways.
Original Music Composed for Films:
Sound effects.
Early video game music would typically use synthesizers.
Now, they use live music throughout the game and have become much more like film music.
Different music for different genres.
Appropriate for the screen action and images it is describing.