This Linux package enables you to download multimedia videos from youtube.com and other selected websites on your debian-installed computer (written in Python, it is also available for Windows and MacOS).

sudo apt install youtube-dl

Update (latest version needed):

sudo youtube-dl -U

To download a video:

youtube-dl http://www.youtube.com/watch?v=DsPYENB88wk

source: http://bredsaal.dk/downloading-youtube-videos-using-debian-ubuntu

To download only the sound in MP3 format:

youtube-dl --extract-audio --audio-format mp3 https://www.youtube.com/watch?v=hy3W-3HPMWg

source: https://askubuntu.com/questions/178481/how-to-download-an-mp3-track-from-a-youtube-video

This also works for other websites, e.g. rtbf.be :

youtube-dl --extract-audio --audio-format mp3 https://www.rtbf.be/auvio/detail_le-journal-de-12h?id=2633236