Music Library Association Best Practices for nature of content

Record the element if deemed useful for identification or selection. Make a note describing the genre or form of one or more expressions embodied in the resource if the expression is not adequately described by the title or by using appropriate genre/form or subject headings. Notes may be constructed stating both the genre/form and medium of performance of musical content, especially if compilations of diverse works can thus be described more concisely.

In MARC: Record as an unstructured description using field 500 subfield $a.