You can record system audio using a freely available app from the Microsoft Store without also getting sound recorded from a PC's microphone.
Download and install Windows Voice Recorder from the Microsoft Store, available here.
2. In Win 10 search for 'Sound settings', and in the Related Settings section, click on 'Sound Control Panel'.
3. On the Recording tab enable 'Stereo Mix' and set it as the default recording option.
4. Disable the Microphone.
5. Be sure to apply your changes and click OK.
6. If you have multiple speakers (or even a second monitor with its own audio system) confirm that the speakers enabled on the Playback tab of the Sound settings, match those on the Recording tab.
7. Voice Recorder mic settings are accessed by clicking on the three dot menu at the lower right.
8. After clicking on 'Microphone settings', scroll down to the section entitled 'Choose which Microsoft Store apps can access your microphone' You should see the microphone setting for the Voice Recorder app set to the ON position, even though the mic has been disabled.
7. Simply click on the big microphone icon in Voice Recorder to begin recording system only audio - only the audio played by the PC. The mic will not pick up any background noise in your room.