Welcome Ed2!
unless you have some kind of hardware encoding device, you'll have to record the sound of your computer while you playback the MIDI (capture the sound) and then save the capture as a WAV file...
Then you can convert the WAV to MP3 using encoding progz.
you are using synths, so you probably record your songs as AUDIO sometimes to put them on CD.. I really thin you should go as usual, and simply convert the WAV recordings to MP3s manually
Good luck.
PLEASE HELP FP BY CLICKING ON
THE 
BUTTON
