Thanks to those who responded with guidance.  I managed to 'charm' my
postmistress into changing her mind.  If anybody else ever has this
issue, here's a couple of interesting points.

1. Yes, the USPS (as previous posters have noted) does consider player
piano rolls to be 'sound recordings', and hence eligible for Media Mail.

2. The US Copyright office recognizes the use of the circle-P
[performance] copyright notice on musical box discs.  The circle-P mark
is used to protect 'Sound Recordings'.

3. The US Patent Office classifies musical box discs (note sheets)
under classification 84, subclass 161.  This subclass is indented under
subclass 115.  Subclass 115, in turn, reads:

  "Selectors -- The means for determining which notes of the piano,
  organ, etc., shall be sounded.

  (1) Subject matter usually includes a tracker-bar, a perforated sheet,
  and driving means.

  (2) Note. These features are shown in automatic telegraphs and in
  patents in many other subclasses of this class, and are sometimes
  claimed along with playing mechanism, such as in subclasses 24 through
  52.

  (3) Note. This subclass includes also details and optical or other
  attachments to the selector not otherwise classified."

Subclass 115 sounds to me like player piano rolls.  So, according to
the US Patent Office, musical box discs are a subset of player piano
rolls.  (?)

Well, this evidence was enough for my postmistress.  (Actually, now that
I think about it, maybe she just wanted to get me out of her office!)

Thanks to all,
Jack Perron
HensTooth Discs (http://www.henstoothdiscs.netfirms.com/)