These are inexpert jottings!
It may be that the sound card in your PC doesn't have Stereo Mix, in which case you need to record from Rec Playback.
I have this problem with my Dell Precision laptops with IDT HD Audio. When recording via Rec Playback the volume on the recording is controlled by the volume broadcast from the speakers, so if you mute your speakers you don't get any sound on the recording. In the end I attached an external speaker to the headphone socket and was able thus to control the sound in the room without affecting the recording.
An external USB speaker is no good, I tried that but when adjusting volume on it the volume on the recording was also adjusted.
In order to access Rec Playback, you may have to go into your sound settings (right-click on microphone icon in taskbar and select Recording Devices), click on a blank space and tick 'Show disabled/disconnected devices'.
My Debut 'Capture sound' setting is Microphone ticked - Rec Playback selected from the dropdown - System Mixer on the right.
If you do buy an external speaker, make sure it has its own volume control so that you don't have to use your PC's volume control.
I hope that helps, and am sorry if it doesn't.