Thanks Michael,
Ah! I noticed last night that the soundboard (the top) did not have the same curvature above and below. I couldn't use "Offset>Shell". Well, it's solved, same thing for the background. In addition, all parts (80) must be solids (as in reality). I will have to make these pieces in several files and put everything together at the end.

MoI3D is really an awesome design tool.