The piezos each have a zener dieode and resistor. and are connected to the mux-sheild NOT physically to the i2c MCP23017 12c port expander. The mux shield handles the inputs i.e. Piezo and the 23017 handles the outputs i.e. LEDs
http://www.thebox.myzen.co.uk/Hardware/MIDI_Footsteps.html
http://todbot.com/blog/2006/10/29/spooky-arduino-projects-4-and-musical-arduino/
Adafruit (from whom I got the chip) has a library. I posted it and the related question Here: [Updated forum post- fewer typos] Help using MCP23017 & Adafruit-MCP23017-Arduino-Library - LEDs and Multiplexing - Arduino Forum. It helps or at this point it's a start.
This part has been already setup and tested
My Inputs (mux shield) and midi shield for midi output
My senors
Cheers!