The only problem is it's only working with my laptop + MOTU MIDI interface, and not with an Elektron Machinedrum (a drum machine). I confirmed that the Machinedrum is sending the correct messages, and what is weirder is that it works if I feed it's output through the laptop to the Arduino.
Does anyone have an idea of what's going on? Maybe something to do with machine specific formatting?
So you have eliminated the question of codes so it's down to the physical layer. You will probably find that your drum machine isn't providing enough current to turn the opto isolator on so try reducing the 220R resistor to something like 150R.
The other solution is that 100K, that is connected to the base of the photo transistor and can affect the sensitivity and the rise time of the output. To make it more sensitive try making this larger, say 200K. Alternatively you could pre bias this input (with a potential divider) so that it has about 0.5V on it. Not enough to turn it on but sufficient to make the optical induced current tip the transistor over.
With a bit of digging, I found out that the majority of Machinedrums don't send out powered midi, so I have to add the correct voltage.
Any ideas on how to do it, and keep the circuit compatible with all devices? I'm sure it's simple. I'm an electronics bumbler (little knowledge of theory, but I can throw things together).
In that case what the device must be doing is pulling down the MIDI output for a zero and letting it float for a one. Therefore add a pull up resistor from the junction of the 220R and connector up to 5volts. Make this resistor 220R also.
I must say this approach rather negates the advantage of having an opto isolator in the first place. I did consider that this could be a problem in my first reply but ruled it out thinking no manufacture would be that stupid. Maybe all the drummer's jokes are true.