Triggering Sound - Arduino + Contact Mic
I'm using piezzos too, for making art installations, and I used max-msp too, but I don't like it too much. 
It's a whole new world change from the computer to the Arduino. Because it's not only the code, you must deal with electronic components I'm quite a novice too and I don't know how much you know about music and electronic components so maybe I say things wrong or that you already know, but I will say:


AC Adapter


I will use an AC Adapter better than batteries, because the batteries will run out in a couple of hours and in a night club almost for sure you will have access to some power extension. Arduino usually works with 9v 1A, but it'll go for 2A.


Music Shield


And you need a music shield, you can buy a cheap one, but I recommend the Adafruit ones, because they come with a lot of more information, and more people are working with them, but they're usually more expensive. With the music shield you need a SD card. Arduino don't have enough memory so you need a music shield with a SD card to storage the sounds.


Speakers and amps


and finally the speakers, it depends in what kind of speakers are you using, if you're using a big ones like some Yamaha MSP you will need and amplifier,  but if you're using little ones like 8ohm 0.25W with the music shield will be enough


some kind of box


you will need to put the Arduino in some kind of box, and I will secure it in the wall and not in the ground