Want to capture an entire YouTube playlists in .MP3? MakeHuman’s free Windows desktop software Free YouTube to MP3 Converter is excellent for that.
First, find the little cog-wheel icon for Preferences/Settings, and tell it to ‘Always expand playlist’…
Then you just paste the playlist URL, re-order if needed, select the ones you want and download. I love it. There’s a paid “Turbo download” mode, but I haven’t needed it. Other than that, it’s really free and nag-free.
Note that it’s important to check if you have the very latest version of the software, as changes at YouTube break it frequently.
A very useful timesaver for multi-file audiobooks, multi-video conference proceedings, and suchlike, where you don’t need the video element. No need to visit a website service and wrestle with a captcha for each file.
For getting audio files from sites other than YouTube, 9xbuddy is an ever-reliable online tool, though I’ve never noticed an ability to handle playlists.