open audacity, in the drop down box in the top left corner it should say "MME" or "windows direct". Change this to "Windows WASAPI"
directly next to the previous drop down box, there is another drop down box, select the option which shows your audio interface and the "loopback" option.
Press record in Audacity, then press play on the video. Make sure no other sounds are coming from your pc
Much more safe than downloads from unknown sources. You can use this technique to record any sound which passes through your soundcard, youtube, netflix, game audio, etc.