Here's some updates. I tried the usb mode as blogged here: http://txton.net/hoehoe/2010/100511030849.html
Everything works! So I think it might be the sparkfun midi shield's 'midi in' problem(I tested the midi out, seems fine). Either it could be a connection problem or the shield itself... Any thoughts?

http://www.sparkfun.com/products/9595 here is the right link. 404 due to the point in the end.

Anyone please???

Hi all, I'm new here:)

I recently built an Arduinoboy by following the Instructables' guide. I used the sparkfun's midi shield: http://www.sparkfun.com/products/9595. As the shield use analog 0 and 1 I changed the Arduinoboy code by assigning the pins to 2,3,4 for the gameboy cable instead of 0,1,2. The circuit should be right... The LEDs flash and I can switch modes.

However when I tried the mGB mode as the guide said, no sound comes out from the gameboy advance (with an EMS cart). I also noticed that under mGB mode when I hit keys on my m-audio instrument (m-audio send midi signal to Arduino), the LED 12 blinks indicating midi channel one active though no sound. But when I changed to channel 2, no LED blinks, same for the other channels. What did I do wrong? I haven't tried other modes yet. Besides, when I turn up gameboy I can no longer loop back to previous mode, any thoughts? Hope these questions are not too stupid.

Someone kind enough to help me out:)
Thanks in advance!