My friend Salvo expanded his Oberheim - Viscount MC2000 with a DreamBlaster X2 to get built in sound.
DreamBlaster X2 is a MIDI expansion board that has USB and WaveBlaster pinout.
It is available at <a href="http://www.serdashop.com/DreamBlasterX2" target="_blank">http://www.serdashop.com/DreamBlasterX2</a>
Have a look at his youtube video, to see how he did it.