Music as output from Interrupter (optical to audio)

Has anyone worked out a circuit for turning the optical output from an interrupter into audio?
I'd imagine something fairly simple - transistor or 2, couple of resistors (to limit input and output) a photo diode or transistor and a loudspeaker. It would be useful to be able to hear the output to test the midi files or conversion of.
I'd imagine something fairly simple - transistor or 2, couple of resistors (to limit input and output) a photo diode or transistor and a loudspeaker. It would be useful to be able to hear the output to test the midi files or conversion of.