Hello,  After much searching I found the following on the USPS web site
regarding media mail. It is in the Domestic Mail Manual section 473.
I have copied sub sections 2.0 through 2.3.

It is interesting that piano rolls are specifically listed as
qualifying in section 2.2e.  There is no mention of music box discs.
2.2e lists "sound recordings" as qualifying and section i lists
electronically readable media as qualifying in a less than clear
manner.  Possibly, a case could be made that music box discs could
be read electronically but whether they are "sound recordings" is
questionable.  Piano rolls are also not "sound recordings" but they are
specifically included.  Anyone who mails piano rolls and occasionally
is questioned by the post person, should print out this document and
have it ready for reference.

2.0 Content Standards for Media Mail Parcels
2.1 General
Mailable matter meeting one of the specific eligibility standards in
2.2, and that is neither mailed or required to be mailed as First Class
Mail nor entered as Periodicals, may be mailed as Media Mail.

2.2 Content Standards
Only these items may be mailed at the Media Mail rates:

a. Books, including books issued to supplement other books, of at least
eight printed pages, consisting wholly of reading matter or scholarly
bibliography, or reading matter with incidental blank spaces for
notations and containing no advertising matter other than incidental
announcements of books.  Advertising includes paid advertising and the
publishers' own advertising in display, classified, or editorial style.

b. 16-millimeter or narrower width films, which must be positive prints
in final form for viewing, and catalogs of such films of 24 pages or
more (at least 22 of which are printed).  Films and film catalogs sent
to or from commercial theaters do not qualify for the Media Mail rate.

c. Printed music, whether in bound or sheet form.

d.  Printed objective test materials and their accessories used by or
on behalf of educational institutions to test ability, aptitude,
achievement, interests, and other mental and personal qualities with or
without answers, test scores, or identifying information recorded
thereon in writing or by mark.

e.  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.

f. Playscripts and manuscripts for books, periodicals, and music.

g.  Printed educational reference charts designed to instruct or train
individuals for improving or developing their capabilities.  Each chart
must be a single printed sheet of information designed for educational
reference.  The information on the chart, which may be printed on one
or both sides of the sheet, must be conveyed primarily by graphs,
diagrams, tables, or other non-narrative matter.  An educational
reference chart is normally but not necessarily devoted to one subject.
A chart on which the information is conveyed primarily by textual
matter in a narrative form does not qualify as a printed educational
reference chart for mailing at the Media Mail rates even if it includes
graphs, diagrams, or tables.  Examples of qualifying charts include
maps produced primarily for educational reference, tables of
mathematical or scientific equations, noun declensions or verb
conjugations used in the study of languages, periodic table of
elements, botanical or zoological tables, and other tables used in the
study of science.

h.  Loose-leaf pages and their binders consisting of medical
information for distribution to doctors, hospitals, medical schools,
and medical students.

i.  Computer-readable media containing prerecorded information and
guides or scripts prepared solely for use with such media.

2.3 Postal Inspection
Media Mail is not sealed against postal inspection.  Regardless of
physical closure, the mailing of articles at Media Mail rates
constitutes consent by the mailer to postal inspection of the contents.

Best regards,
Spencer Chase