How to flash the GBA-MIDI-cable to recover from corrupted flash or get a newer firmware version:

• Grab a firmware update file from http://meuk.spritesserver.nl/gbamidi/
• Grab something that can play the MIDI file. I know Windows programs sometimes have trouble playing the file: vanBasco's Karaoke Player usually works in that case.
• Hook up the GBA cable to a GBA and a MIDI port.
• Turn on the GBA, and at about the same time start playing the midi file to the cable
• Wait 30 seconds or so.
• If everything has worked, the GBA should boot into the firmware again. If not, try turning it off and on again.