I have no idea how this is possible, but it is awesome. Actually here's how it's possible:

The MIDI files have to be edited a bit to be played on the printer: some channels need to be disabled (percussion stuff), some are transposed to avoid exceeding the frequency limit. Also the volume of the individual instruments never fits when the original settings are used.

Now if some nerd could translate what that means, that'd be great. [via MIDIdesaster]