Ensure you follow the steps below carefully as there is some advanced troubleshooting that should be performed with care.
If the audio device inside your computer is a Conexant HD Audio device, the steps below might help you restore Stereo Mix inside Windows 10.
Right-hit on your headphones in the list and pick Update Driver from the context menu.
Windows 10 may have somehow automatically impaired the Stereo Mix feature as a virtual sound device.