If you can't source a sound card, I have a 2 port game card you can have. That's really the way to go, for a Compaq III. Find any old IDE soundcard and the associated DOS drivers. The old IBM game adapter is for the IBM PC, no point in doing that when there is a much easier and cheaper option. Search Ebay or craigslist for things like
Creative Labs Sound Blaster 16 Bit ISA Sound Card
Just got off the phone with Jason P. -- he's got a NIB two-port game card to donate -- I agree that it would be even better to have a combo sound/game card. Next time I'm door there, I'll find out what if any cards are already in the computer + what we might have in other systems in the warehouse. Bottom line, now I have a project for the next workshop :) and I'm feeling optimistic about getting the computer configured exactly as we need it.