Electronic drums design

I'm trying to design and then build an electronic drums kit, but i have some problems, first i need to figure out a way to make a microcontroller to output audio files from an sd card, i thought about using the adafruit wave shield but it doesn't allow to play multiple files at once, and next i need a way to figure out how to tell the microcontroller which drum I'm hitting so it can read the piezo sensor, i was thinking about using interrupts but then i would need 8 interruptible pins.

i was hoping someone could help me with that, I haven't decided yet what microcontroller I'm gonna use, but i doesn't have to be strictly an arduino, I could learn how to code for other microcontrollers also.

Thank you!