Licensing agencies, also called Performing Rights Organizations (PRO's), have repertoire lists for the works that they manage the rights. If you want licensing to perform a work, arrange a work, or record a work, try searching these PRO's repertoire lists to find out who to contact.
Overview of music copyright
How do I find out who owns the rights to a musical work?
How do I request permission to arrange a musical work?
What Musical works and sound recordings are in the Public Domain?
What is allowed under educational use (fair use) for music?
How to Register Your Copyright
Information on Copyright and Audio Preservation