Which software can be used to extract MP3 sound from both Facebook videos and YouTube videos?
It must run on Windows and be gratis.
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Youtube-DL (https://rg3.github.io/youtube-dl/) should fit the bill.
Note: To use option
--extract-audio, you need to install ffmpeg, too. Just download an exe and put it into the folder of youtube-dl. See e.g. https://satuimrambles.wordpress.com/2016/09/27/how-to-install-youtube-dl-ffmpeg-and-rtmpdump-on-windowsmaclinux/ for details.