Hooked up everything to a breadboard and ran some preliminary tests using the Arduino IDE to check connections and functionality. So far so good! The 8x8 matrix and teensy Audio shield share the same I2C bus and everything seems to work fine. I was able to get the device to generate some audio waveforms while also displaying some simple images on the matrix (what a relief). 

I've also decided that I'm going to go ahead and try to crank out a PCB breakout for all these components before the submission deadline. It should be pretty straightforward, but I'm a little bit concerned about the lead time. If I can get this thing sent out by the end of the day tomorrow, I think I'll feel pretty good. I've marked out all the pins I'll be using and started creating my EAGLE library. Between the pins being used by the Audio shield and the pins I'll be using for my additional components, I have a single free pin left (17), pretty lucky!