I've got some news. It appears that it's mux no4 that creates the problem. That narrows thing a bit, but I noticed something else. Every hit on my test piezo will result in one proper note (EZ drummer will produce the right sound) followed by many false notes (no63 played by EZ drummer as hihat hits). Here's a pic:
http://imageshack.us/f/811/clipboard01js.jpg/
My test piezo is a 27mm Murata piezo wired by me with two 50cm single coil cables, with no resistors at the moment. For the time being, I have removed the 4th muxer.
Please help people... What are all these 63notes appearing like crazy?