WASAPI loopback should be available on Windows 7 and later. With loopback you should get the original audio with no volume changes or bass/treble adjustments or any other effects applied by your soundcard drivers. This is usually the best option and you can always Amplify or Normalize after recording.
Stereo Mix or What-U-Hear are not available on all computers, but these will be affected playback volume and any other effects.
Many of the streaming services are now automatically volume matching so that all songs/programs are approximately the same volume. I Believe YouTube is doing this too.