Harry Houdini left some of his property to the Library of Congress when he passed away and you can too. The Library states that "bequests have been the lifeblood of the Library's growing endowment." If you want to support the Library of Congress by leaving your copyrighted music to it, you can do so by specifically designating your wishes in a will.
Draft a will according to the laws of your state. Generally, you need to name someone to be your executor -- the person designated to carry out your wishes. You also need to state that you are of sound mind and body and creating the will of your own free will.
Write "I give, devise and bequeath to the United States Library of Congress, Washington D.C., all rights and interests in" followed by the copyrighted music that you are leaving to the Library. Describe your music in as detailed a way as possible. Where possible, include song titles, composition dates, recording dates and copyright registration numbers.
Write down how you want the rest of your property to be distributed.
Sign the will. In most states, this must be done in front of two witnesses.