The project is still in early prototype stage. The current design uses a 30 cm square aluminium sheet as the active surface, with piezo sensors mounted under each corner to pick up the pressure pulse caused when you hit the pad with a stick or your fingers. The pad is supported on closed cell foam rubber discs, inside a plywood frame. When you hit the pad, the sensors in each corner respond differently according to where you have hit it.
The signals from each sensor are captured by an arduino
micro-controller board, which then sends the raw signal strength from each sensor to a computer over its usb cable. When the sensor data reaches the computer, it is interpreted by a python program which calculates the coordinates and velocity of the strike from the raw figures, and then maps this information onto midi note and controller values.
What happens next is up to you - if you want, you can feed the midi data into any Linux softsynth just as you would for any other midi source like a keyboard. To make the most of the position sensitivity of the pad, though, you want a synth that will change the sound produced according to the x and y position of the strike. The simplest way of doing this is to use one of the music programming languages like supercollider, csound, or pure data. Some example supercollider synths are in the archive attached to the final step of this instructable.
Unfortunately I forgot to take proper photos of each step as I was building the pad - the ones that I have attached are really assembly photos that show the main parts before you bolt them together. They should give you enough of an idea though to build one yourself.