To enable Stereo Mix on Windows 7 make sure your Audio device drivers are properly installed and updated, there are some cases that users may find that everything works fine however they couldn’t see the Stereo Mix on their sound properties and one of the possible reason is that the device driver
configured on that device is a generic driver.
With “Stereo Mix” enabled, you can open up your most loved recording programs and select that rather than your microphone before you record. In the event that you don’t see the alternative, or your program doesn’t give you the chance to change your recording device, you can deactivate or unplug your microphone and then select “Stereo Mix” as the default recording device.
To enable Stereo Mix right-click sound icon at the lower right corner of the screen then select Recording Devices.
Under Recording tab right-click anywhere on the selection devices the select Show Disconnect Devices.
Then it will display Stereo Mix.
Right-click Stereo Mix then select Enable, click OK or Apply to save that changes.