Windows Audio

Stereo Mix
The Stereo Mix loopback recording feature is sometimes disabled on standard Windows installtions. This can sometimes be remedied by installing the driver from the vendor instead of the default driver supplied by Microsoft.

In the Windows Device Manager on my Dell machine, the audio device was listed as 'High Definition Audio Device' with no mention of a manufacturer. I looked in the device properties, and found this hardware ID:

HDAUDIO\FUNC_01&VEN_10EC&DEV_0899&SUBSYS_102805B7&REV_1000

The vendor ID is 10EC, which is Realtek, and device ID is 0899.

I went to realtek.com.tw and found the driver for their High Definition Audio Device and installed it.

After that, Stereo Mix appeared in Recording devices.