The Arduino software has a terrific MIDI signal library created by Francois Best, so it wasn't tough to convert the output from the photoresistors to a MIDI signal. The output of the 4 bit multiplexer was connected to one of the analog input channels of the Arduino. The program checks if a beam is being blocked (by checking for a voltage change read by the analog input channel), and then creates the MIDI signal (note and velocity) depending on which beam was detected to be blocked. To fetch the code for creating these signals download the Francois Best MIDI library and check out the link above. You also need to get a MIDI jack and connect it as shown in the schematic. Check here for more info on connecting the jack, but I really suggest the MIDI library I have listed above.