If you still have a keyboard with a floppy disk drive, this is the best way I know of to record those midi files and change them to MP3s. I posted a detailed set of instructions at
http://psrtutorial.com/Lessons/L_Recording/LPR-MakeCD/lpr-makecd.htm Goldwave is a shareware program that is rated as among the best for recording.
Enjoy,
Gary
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)