I understand you want to use the stuff you have lying around but I thought I'd just send you this board because its a small, fast, cheap and easy to develop stuff like this on.
I can imagine you can generate the ADSR on the Teensy too, like they did on a similar IC, the Microchip PIC here:
Maybe a few IC's which can help you interface other logic to the MPA board you have (or maybe it's on there I don't know):
The ULN2003/ULN2008
darlington arrays which you can connect motors/actuators/relays on (maybe you need to trigger a sound that way)
or HC4066 quad analog switch
image-Google around for them and see what people use it for.
Sorry it's a bit fragmented tips but maybe it will you get inspired so you can solve a piece of the puzzle you're looking at.