Stereo Mix can be enabled and accessed on Realtek and perhaps others. If you want to record streaming video using such software as Roxio or Sony SoundForge, this input must be selectable.
I have HP Pavilion with Realtek integrated audio. Try this.
1. in Control Panel, select "Sound".
2. Select "Recording" tab. You should see "Recording Devices" icons, probably Microphone, Line-in and maybe Digital In.
3. If you see Stereo Mix, good, follow step 5. But it seems to come disabled on HP iron.
4. If you do not see Stereo Mix, right click in an open spot of the dialog box (ie., not on an icon) You should see a choice " Show Disabled Devices." Select it.
5. You should now see Stereo Mix and an icon of a sound card. If it is grayed out, it is disabled. Right click on the icon.
6. You should see a a choice "enable" click it and you should be functional. You can also open "properties" if need be from this right click menu.
Gripe: Neither HP nor Realtek have an FAQ on this hidden functionality. I found a tutorial on YouTube or I'd never have discovered this.
I'm running a Asus P6T w/Realtek HD Audio and Roxio Sound Editor v10 won't let you select a input source to record streaming audio.