Not sure if anyone made the point, but the PianoDisc system does,
indeed, use MIDI.  You can convert Internet MIDI files to play on
your piano.  Following is a quote from their web FAQ at
http://www.pianodisc.com/info/faq.html :

 "There are a huge variety of MIDI files available on the Internet
  from many different sources.  Some files will play on your PianoDisc
  system without modification.  However, most will require a couple of
  minor changes.  First, you need to make sure the file is Type 0 (Type
  1 will not work).  Second, you must set the instrument to "Acoustic
  Piano" or "Bright Acoustic Piano" on the channel that you want the
  piano to play.  These correspond to General MIDI instruments 0 and
  1 (or 1 and 2 if you do not have instrument 0).  The best way to
  modify files for use with your PianoDisc system is from a MIDI
  sequencer.  A wide variety of sequencers are available for both
  Windows and Macintosh computers.  See your local music retailer for
  more information on a sequencer that would meet your needs."

Philip Jamison
West Chester, PA