Meaning, if you check the checkbox, you say to Windows itself that you want to monitor what is played through this connector/channel, and also in what you want to hear what you will be monitoring (speakers or headset. This tab must be understood as playing the same role than the monitoring process in a DAW software or sequencer. Thus then, you will see more tabs in the properties window. ) actually plugged into the Line In connector of your soundcard.

It means you have to go in the properties of this device, BUT you have to do this with something (it can be anything, including a speakers' or headset's plug.

The problem was due to the configuration of the 'Line In recording device' in Sound properties. (nothing to do with audacity, by the way).

Granted that your soundcard device is fully recognized in Windows, activated and all that, and that you wonder why nothing is detected on the stereo mix when something is playing on your computer. Ok dudes, I had the same problem and I finally found where the glitch was coming from.

Skype can t detect sound card