Hi there...
Watch out when using other people work...I dont know about songs but my hunch is that if it has a copyright its not free for you to use. Even using other peoples characters is a touchy subject. The parent of one of our students is a screenwriter, we asked if he could write something for us using two of his characters, he said no, he actually doesnt even have the right to use them anymore. They belong to the production company now.
Parody and Farce are two areas where you can use other peoples characters or music.
Just be careful. When in doubt dont do it. You never know how far the ripples in your pond will travel.