So you went to the Recording Devices and allowed showing the disabled devices. You then enabled the stereo mix, which should have made that source available.
Are there any settings on the Configure or Properties buttons that might help?
Does Audacity need to be set to see the new source?
There are some communications settings on the Sound dialog, but they normally deal with headsets with microphones with which you can communicate.
Do you have any dedicated Sound Mixers in the Control Panel, like a Realtek utility?
Since I don't have Audacity, I can't really give you a definite answer.