I've submitted the following via the Library of Congress's Suggest terminology form:
I'd like to see a heading for Mix tapes, and another or a cross reference for Mix CDs.
I recently had a scholar at Barnard College who was researching mix tapes. A search in your catalog yields 35 results, (in mine/Columbia University's it yields 82), including one in particular that could serve as a good warrant:
Mix tape : the art of cassette culture / edited by Thurston Moore.