Catalogers working with music materials will want to be current with the Music Library Association’s Best Practices for Music Cataloging Using RDA and MARC21. Since February of last year, this document has been integrated with the RDA Toolkit (under the Resources tab).  You need not be logged on to the Toolkit to access Best Practices, but the links to RDA instructions will not work unless you are.

If you want to consult a more traditionally formatted document, the old website of the former Bibliographic Control Committee of the Music Library Association has links to PDF files of Version 1.1 (the final version issued as a complete stand-alone PDF file), a list of changes between versions 1.01 and 1.1, and Supplements to Best Practices for Music Cataloging Using RDA and MARC21, version 1.5, 12 April 2016 (this last document is also available at the current website of MLA’s Cataloging and Metadata Committee).  Be aware, of course, that the Toolkit-integrated document will presumably be more up to date.