Disklavier

Here's a product designed to convert MIDI to mp3.  It is
modestly priced, and there is a free demo for testing, but it
looks like the demo may only convert to .wav, rather than mp3.

http://www.midi2wav.com/download.html

And a bit of the description from their site:

Midi2Wav Recorder (ver 3.61) is a program with the ability to
convert MIDI to WAV files and MIDI to MP3. 

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