Musical scores are treated the same as referencing a book.     

Format:  
FAMILY/ SURNAME, Initials. (Year of publication – in brackets). Title of score – in italics. Series title and volume – if available. Edition – if not the first. Place of Publication: Publisher.     

Bibliography example:  
NYMAN, M. (2003). Pieces from the Piano (Pocket Manual) (Sheet Music) UK: Music Sales Ltd.   

In-text citation example: 
(Nyman, 2003).