Hey,  I know you focus on the classic mechanical music boxes
(which I think are really neat, by the way), but I have recently
succeeded in making a custom music box out of digital electronics
(microcontrollers).  I have released my source code under the GPL,
so if any of you want to use it should be less painful than developing
from scratch.  You'll probably want to clean up my code tremendously
if you wish to make a commercial product, but for a hobby project
it is just fine.

  http://www.instructables.com/id/Poor-mans-AVR-soundcard/

Once you have the necessary equipment, which costs under 80$
(I used an Atmel STK500 programmer), these devices cost from $4 to
$10 each to make in small quantities.

Sean Boyce

 [ AVR stands for "Advanced Virtual RISC" microcontroller I.C.
 [ Several interesting projects are described at http://attiny.com/
 [ -- Robbie