Comment by zuck9

  > youtube-dl -F OKbtC223e30
  139         m4a       audio only  DASH audio   49k , audio@ 48k (22050Hz), 525.88KiB (worst)
  171         webm      audio only  DASH audio  129k , audio@128k (44100Hz), 1.27MiB
  140         m4a       audio only  DASH audio  129k , audio@128k (44100Hz), 1.36MiB
  172         webm      audio only  DASH audio  176k , audio@256k (44100Hz), 1.78MiB
  141         m4a       audio only  DASH audio  255k , audio@256k (44100Hz), 2.71MiB

Just use youtube-dl -f 141 to direct download just the audio from YouTube. I always use this to download songs.