Holoman, D. Kern. Writing about music: a style sheet. 3rd edition. University of California Press, 2014.
also available in print, Music Library Reference ML3797 .W75 2014. The go-to source for citing music materials-- online recordings and other media, CDs, blogs, Facebook, Twitter, and so on. Also how to prepare notated examples, create concert programs, handling foreign languages, where to hyphenate Beethoven, best practices for student writers, and etc. Essential.
Citing Music Sources Using Turabian/Chicago
Includes info on how to cite online resources, including recordings; maintained by Indiana University
Citing Music Sources in your Essay and Bibliography
maintained by the University of Western Ontario