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
http://www.videopete.com/midi/ 

Dick Partos