The Publication manual of the American Psychological Association has never expressly addressed music scores. Citation and referencing of such materials has always required authors using APA Style to proceed by analogy, seeking guidance from the examples relating to books and book chapters, periodicals, or archival documents as appropriate.
In anticipation of practical need, some preliminary guidance on the treatment of music scores is now provided in Jeff Hume-Pratuch’s “How to Cite a Musical Score,” posted on the official APA Style blog (2011, September 29).
While not an official source, Sampsel (2009) provides concise advice on the use of APA Style in writing about music.
Sampsel, L.J. (2009). Music research: A handbook. New YorK, NY: Oxford University Press.