the main thing to remember is what exactly is what.
midi isn't sound - it's data that your soundcard decodes and turns into sound. if you use midi data in a daw (digital audio workstation, like reason, fl, or reaper), your program is converting that information (pitches, velocity, patch data) into data so that it can put into a vst and get sound out.
wave/mp3/ogg/whatever IS sound that your computer just spits out.
so, your question of midi to mp3 isn't actually possible - it's like taking scrap metal and turning it into canned food. you have to turn the metal into cans, and then put food in them - there's a step in between.