I have found a way to convert a midi sound file to an mp3 for free. Here is how I did it.

You'll need an app that will record sound, windows media player and a stereo patch cable with two male ends.

1) Plug one end of the patch cord into the output of your sound card and the other end into the mic/input
2) Start the app that will record sound with the record option running
3) Start windows media player and play the midi file
4) Switch to the recording app and monitor the levels.
5) When done, remove the patch cord and edit the recorded sound.
6) Then mix the recorded sound down to an mp3.

I take no responsibility for any damage you do to your computer while attempting this. But, it worked for me. I was a little worried about patching one side of the sound card to the other side but it worked okay.