Francoise:  It is doubtful that the crack (split?) in the sound board
will impact the quality of the music on your Polyphon.  However, a
potential problem may be buzzing if the board becomes loose.

I would guess that the back with the wood panels (the grain of the
wood) is always up and down, and not crossed like plywood, for the
simple reason that to make wood panels in the other direction would
require wider trees.  It is very likely not done for any sound box
effect.

I'm not sure what you mean by "the two large wood pieces in the back."
Perhaps you can explain how these two pieces of wood are used in the
construction of the box.

Musically yours,

Bill Wineburgh
http://members.aol.com/mboxmuseum/museum.html