Eventually you will need to extract audio from the video files you downloaded with Youtube-DL.
If you face this error :
[youtube] xxx : Downloading webpage
[youtube] xxx: Extracting video information
[youtube] xxx: Downloading DASH manifest
[ffmpeg] Destination: xxx.mp3
ERROR: audio conversion failed: Unknown encoder 'libmp3lame'
Be sure to install
libav-tools (tested on Ubuntu) and you're good !