Very nice work, Cooly. (Coolie is Italian slang for butt, by the way. It's the equivalent of "toosh". Just thought you should know)
I was planning on making THE recording of this piece. When the composer decided to download a DAW and a piano library himself and figure out how to make recordings without any help, which impressed me, I offered to back out and let him do his own thing. He asked me to finish my version anyway, and so I strayed from the original blueprints a bit. That's why his version is dreamy and ethereal, while mine is biting and impassioned in places.
As for my process... it's not nearly as simple as "a converter", though that sort of solution exists.
First, I imported Cooly's MIDI performance into Cakewalk Sonar, and made edits to most of the chord voicings throughout the piece. I also smoothed out some of the jerkier passages by moving notes and/or adjusting the tempo on the fly. The most notable change I made was to even out the rhythm and glissandi of the intense part at 3:08. Then, I auditioned a few piano sample libraries to see which one was the best fit for this piece. I picked the "Steinway B" instrument from East West.
Next, I made my way through the piece, measure by measure, adjusting the timings of the sustain pedal on/offs to best fit the sound of the samples. I followed that with note-by-note velocity edits to make the performance as diverse and musical as I could (This is often called "humanizing")
That just leaves production. (For the record, all of the plugins mentioned come out of the effects suite prepackaged in Sonar, called "Sonitus".) First, I manually scanned the piece with a parametric EQ to pick out some trouble frequencies, and lightly cut them. I also added a minor low-cut shelf out of habit and perhaps superstition.
Since most of the piece is on the quiet side, except for the aforementioned intense section, I opted for a small amount of compression to enable the rest of the piece to be audible without the intense part breaking eardrums. The final touch was some "Concert Hall" reverb. The preset in Sonitus is pretty decent, but I always tweak it substantially just to feel like I have 1337 skillz.
@Lot: What programs are you already familiar with, as far as digital music is concerned?