It's not so much a sound editing app, tho, you can alter the instrument set used and export the sound to WAV.
It's a player that plays MIDI in its most original form possible , for OLD midi. So for old game MIDI's, particularly like Duke Nukem, DooM etc, this player tries to emulate older hardware and plays it using similar instrument sets. The difference being rather than the relatively "sophisticated" instrument set in Windows 2000 XP, Vista, etc, this emulates a very basic set so your MIDI will sound a lot different.
This is for positively oldschool people!
This is free to use.. but it took me AGES to find, so I thought I'd torrent it for the benefit of you all.