Help with an eDrum

hello all,

i am currently working on building my own eDrum 

(http://www.geocities.com/edrumbrain/index.html)

and i realized that the device only converts the input into a MIDI
stream. Is there an affordable way for me to convert my MIDI into
audio on the fly? (To, say, be able to plug the drum set into
headphones/guitar amp)

Any/all help is greatly appreciated!