Good read on the issue
here

If you are under any delusions thinking that it is OK because you are only doing music that is public domain. . .

QUOTE:
technically is no such thing as public domain, since arrangements become property of their arrangers. This is certainly the case with centuries-old classical and folk music, whose copyrights have long since expired, but whose performances are being regulated by ASCAP and BMI as copyrighted arrangements. In fact, ASCAP has over 40 arrangements of Beethoven's Moonlight Sonata on file, and even more amazing, nearly 80 versions of Row, Row, Row Your Boat!