I figured I'd post this in audio since the main problem is integrating the MP3 shield.
I started by working on the instructables code here: http://www.instructables.com/id/Use-an-Accelerometer-and-Gyroscope-with-Arduino/
I'm using the Single Pixel sketch near the bottom of the page as a base. I'm using a shift register with a 4x4 led array. It also sends midi out to the computer to make sounds. This works fine and dandy on the UNO.
Since I didn't have the accelerometer and gyro board she uses in the instructable, I picked up an MPU-6050. I used the I2C dev library here: http://www.i2cdevlib.com/devices/mpu6050#source
The combination of the two works well on an UNO. But I want to generate the MIDI via the arduino and not on the computer so the device can be liberated!
I picked up a sparkfun MP3 Player Shield: https://www.sparkfun.com/products/10628
I found a sketch that enable the shield in "real time midi mode". On it's own I can get it to play midi.
The problem is integrating the midi shield code into my existing code. I switched over to a MEGA 2560 so I had more ports to play with, the MP3 shield takes many. I can get the leds working on the shift register, the I2C working for the gyro/acc and real time midi mode working on the shield, but when I combine them... No go.
I'm new to using ports and interrupts. I'm fairly sure I'm having some kind of timing or interrupt problem but I'm not sure.
My code is here: https://gist.github.com/4439033
If anyone has ideas on how to make all this jive, I'd love to hear it!