MIDI suggests that you have another MIDI capable synthesiser or something playing the notes. If that is going to be an Arduino playing sounds via an mp3 player you probably don't need to convert the notes to MIDI at all.
But I'm not sure it will work very well. In my experience an mp3 player finding a "file" on an SD card and starting to play it takes quite a long time, maybe several seconds. There would be quite a delay between working out the note and hearing the sound which is probably not what you want for an instrument.
You might have more luck using something like Fluxamasynth Shield v.3 – Modern Device or one of the other Arduino synthesisers to create the sounds.
Steve