I would say that piano rolls still qualify for Media Mail in two ways:
(1) they are certainly recorded music, but for all the youngsters
working for the Post Office, they are (2) computer-readable media.
The player piano was the second external data storage media computer
system,  the first one being the spinning jenny which wove cloth in the
mid 1800's.

D.L. Bullock  St. Louis
www.thepianoworld.com