Download and install Audacity.
Go to Edit > Preferences… to open the settings.
Go to the Devices tab on the left.
From the Interface section at the top, change the “Host:” option to Windows WASAPI.
From the same window, in the Recording section at the bottom, change the “Device:” option to be the output device, like your speakers or headphones.
Click or tap OK to save and exit.
From a web browser (doesn't matter which one), open the video you want to “convert” to MP3, and then be ready to hit the record button in Audacity as quickly as you can.
That, or you can start recording in Audacity first and then start the video, but then you may have to do some editing in Audacity to remove any silence at the beginning.
Hit the stop button in Audacity to stop recording.
To save the recording to MP3, go to File > Export > Export as MP3, and save the MP3 somewhere you can find later.