Music intended to be performed before and during a play to set the mood for scenes or highlight dramatic action is known as

It's an incidental music

Explanation:

This kind of music is used to create a mood about a moment or a character. Because of that, it's something that enhances the story (stage play or motion picture). With the development of the leitmotiv by Richard Vagner, incidental music became stronger, turning into a signature of the art that purposes to enhance.