Your computer plays a midi file through to your sound card. Your computer is thus translating a digital signal into and audio signal. Your recorder can probably record the audio signal from the sound card out jack, thus your recorder ends up with an analog sound recording if it is a tape machine or a wave file if it is a hard disk recorder. If your tape machine reads MIDI files, which is less likely, this is a matter or copying the midi file from your computer hard drive to the recorder hard drive. Use a floppy disk or network to your recorder. I have no idea of the what kind of a recorder the MU4 is, but I would assume that you are trying to make an analog tape or a wave file, as your computer plays your MIDI file. If you can listen to your Midi file on your computer sound card via speakers or earphones, than they have the necessary signal to make a recording. A basic mini stereo jack out from you sound card to the line in input on your recorder should get that signal to your record. Monitor it with your recorder earphones to be sure its coming through. If the signal is coming through and your recorder is not broken, you should easily be able to make your analog tape or your recorder hard disk wave file, whichever type your recorder does.