The USPS Domestic Mail Manual is quite clear about music rolls
qualifying for Media Mail rates: "... these items may be mailed at the
Media Mail prices: Sound recordings, including incidental announcements
of recordings and guides or scripts prepared solely for use with such
recordings.  Video recordings and player piano rolls are classified as
sound recordings."

I used to have discussions with the clerks frequently, until I began
attaching a label to each package stating "Media mail per DMM 173.4.1.e
- contains piano rolls."  There has been no problem since.

Mike Schoeppner