Few month ago I investigated on the same topic and I found you same solution and same issue. For the time beeing I've the same connection done by AdamK47
(Pin 9 and 7) with only an addition of few resistors to decrease the voltage (from 5V to 1V): when I did the connection I wasn't sure of nvidia GeForce 8800GT datasheet.
, I can hear only Stereo output. I tried to format PC, installing XP and latest version of the X-Fi driver but no news on the topic.
In the next days i want to test using KMPlayer and YPax driver. In case of success I'll update you.
Frankly speaking, I'm not sure if problem is hardware or software. Does anyone have an update on this topic?