Hi Joe, take a look at my calliope at http://rpartos.org/calliope.html
I'd be glad to offer you any help in building a similar instrument.
As far as the MIDI interface goes, it's simply a matter of using any
software MIDI player (I use Winamp) outputting via a USB MIDI cable to
a hardware MIDI decoder in the calliope. The decoder controls the 44
solenoid-operated pipe valves. The decoder I use can be seen at
http://www.j-omega.co.uk/mtp8.html and more such boards are listed at