Jack,  The authoritative document on the post office web site is at
http://pe.usps.gov/text/dmm300/373.htm#wp1015205  See particularly
300.373.2.2fe: "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."

Now that doesn't directly talk about music box discs, but elsewhere
(such as http://postcalc.usps.gov/PopUps/pop_up27.htm ) they define
Media Mail as including "computer-recorded media like CDs and diskettes",
which are certainly non-paper, digital, and often contain music.

I send roller organ cobs all the time, with the approval of my local
post office.  If all else fails, can you perhaps go over her head and
get a ruling from whomever governs your region?

Todd Augsburger - Roller Organs
www.rollerorgans.com