I have been planning this design for about a year now, and have been actively working on it on and off for about 8 months. Much of the design is completed: the analog portion (peak detection, ADC reading, and peak detection reset) has not really changed for 6 months. The digital parts have been going over iterative changes: initially I was planning on using a Raspberry Pi for playback and an AVR for ADC, but ended up deciding against it due to boot up time and overall complexity (who wants to run apt-get upgrade on your drum set!?!?!). I then started to look at STM32F4 chips, which look quite nice but and could doubtless handle the job, but it was a big learning curve to move to ARM and immediately start doing audio. I ended up choosing to use the Teensy 3.1 plus the PJRC audio board; the audio library which runs on this hardware is very capable and yet is easy to use, and the Teensy can handle the ADC parts as well as the sampling, which means that I only need one microcontroller.

I have the physical hardware design completed and have started work on manufacturing the actual drums. (I am re-doing them rather than using my previous ones since I want to reduce the weight on this set).

Everything is currently on a breadboard and working. I have 12 sound polyphony (i.e. 12 sounds can be playing at the same time), which is more than enough (my self-made spec called for 8).

I am going to be doing a bit more prototyping, and hope to have the PCBs ordered within a week or two. Then it is just a matter of finishing up the software and drum manufacturing.

Please feel free to ask if you have questions.

Cheers