I think you need a SPDIF / Toslink connection on your speakers and a cable, which you use with the HD connector cable
I'm not sure about adaptors really, I very much doubt you can get digital to analogue audio leads (and if you could it'd be pointless) but I could be wrong.
I think the only other way you can have audio is analog by red and white connectors which will plug into your speakers and may provide Dolby ProLogic II
If he only has the 3 connectors for PC you're gonna have to get an adaptor like this
http://www.logitech.com/index.cfm/pr...ONTENTID=10050
but this will only simulate surround sound, it will not give true 5.1 positional audio
Hope this helps