Cite sheet music like a book (or chapter, etc., as appropriate). If the title is in a language other than English, include the original title with the translation after.
Last Name, First Name. Title of the Piece. City, State: Publisher, Year.
Beethoven, Ludwig van. Moonlight Sonata, Op. 27, No. 2 (Complete), ed. Willard A. Palmer. Los Angeles: Alfred Publishing, 1986.