Only use the narrator as necessary. Have the characters tell the story themselves. It is boring to have the narrator tell the story. You might as well just read the script if you do have the narrator talk too much. A crash of music used for special effects, strong emotion, humor, etc.
Background music is usually classical music.
In your groups, write a radio drama of at least 5 minutes in length and not more than 10 minutes in length. Be sure to follow the above directions to write a good play.