A mystery film means there will be lots of suspense and intrigue. This means that the movie has a serious tone, and involves scenes that are uncertain yet interesting, with new developments and unexpected things.
So, the background music for a tense scene in such types of films will be something that will add to the suspense and help create a more mysterious atmosphere. Playing intense and dramatic background music will help in creating an atmosphere that will captivate the audience's attention. If the music is light-hearted or playful, then the seriousness or suspenseful nature of the movie may mean nothing.
