-f 140 is the right yt-dlp parameter to get just the audio.

I think it's just "-x". It should select the best audio quality automatically.

Oh, I missed that! In that case,

  yt-dlp -x <url>

.. or optionally to convert it to mp3 on the fly:

  yt-dlp -x --audio-format mp3 <url>

.. or doing so with also the best audio quality available:

  yt-dlp -f 'ba' -x --audio-format mp3 <url>