I seem to remember in the late 1960's that Q.R.S. Music Rolls got the
U.S. Post Office to classify player piano rolls as sound recordings.
Back then the rate was "Fourth Class Sound Recordings".  The Music
Trades ran an article on this as well as a letter from Q.R.S. to the
roll dealers.

My local post office has given me trouble and refused to mail piano
rolls as Media Mail until I went to the Postmaster and made them pull
the regulation book.  It is in their book that piano rolls are sound
recordings and are therefore qualify as Media Mail.

Don Teach - Shreveport Music Co.
Shreveport, Louisiana