It can happen on Realtek sound cards, for example, the input for Stereo Mix does not appear.
In reality it exists but has been disabled.
- Go to Control Panel -> Classic View -> Sound -> Recording Tab -> Right click on the list -> Display disabled devices.
- The StereoMix icon shall be displayed in the list. Click on it.
- In the window that opens up (Device usage) select "Use device"
The principle is the same as for Windows Vista, except that the control panel is not quite the same.
- First, double click the speaker icon in the taskbar.
- A mixing interface should appear. Click on Options/Properties.
- Select the Recording Tab and check the Stereo Mix option(Stereo Mix).
- Turn on the mixing interface and check the Select the Stereo Mixer option.
- Click on OK to finalize the operation.