At first, when I read this question, I double checked the date as it
seemed like an April Fool's Day question, but when I see the answers,
I can only believe that it is truly a serious question and people want
to know the truth.  If we look at how a musical box makes its music,
we have the following ingredients:

1. The program being played -- in this case, it's the disc which
   operates the star wheel gantry plucking the note;

2. The tuned steel comb;

3. The case, the sounding board, and the connection between the
   sounding board and the steel comb.  (There is a chamber between
   the two, we can call that the case).

The sound is produced by the vibrations of the musical comb, the exact
frequency is tuned by the craftsmen of old, assuming that it's a double
comb musical box (as the majority of the disc playing musical boxes
are), the two combs were tuned to slightly different frequencies, and
that difference increased the volume of the notes.  Then the case, or
in a larger musical box, a post would press the baseplate to the sound
board, transferring the vibrations to a wooden board, filling the sound
chamber and resonating throughout the room.

Where does the disc come into the equation?  It is held tightly against
the star-wheel gantry by the rollers (and there are opposing rollers as
well  keeping the surface flat and tight).  No vibrations means no
sound.  I do have several _old_ discs that may have lost some of their
elasticity and they creak when being bent by the rollers, but it's not
part of the tone.

Physics tells us that the disc material has absolutely _no_ effect in 
the tone of a musical box.

Eli Shahar
Sonora, California