Update because I think no one thought it was easy. Just read it if you want to use it for private use (you know the terms youtube and the like).
After doing some research with various APIs and libraries and studying the source code, I just tried this on my own and wolla.
Simply put, only audio links are in the source page of each video.
You just need to know what you are looking for;)
Open the youtube video, browse the source of the page and find "mime% 3Daudio". There should be 5 matches, full URL, for example:
"HTTPS% 3A% 2F% 2Fr2 --- sn-35cxanpbo5a-8pxl.googlevideo.com% 2Fvideoplayback% 3Fsource% 3Dyoutube% 26signature% 3D30EC556F55533FBFD9003767730D10556681F33A.B12E021CA54CCB9E225F58A4430E9BB528081FB5% 26requiressl% 3Dyes% 26expire% 3D1527306011% 26clen% 3D3064602% 26initcwndbps% 3D1152500% 26ipbits % 3D0% 26mime% 3Daudio% 252Fmp4% 26dur% 3D192.911% 26fvip% 3D5% 26lmt% 3D1524946334873350% 26key% 3Dyt6% 26sparams% 3Dclen% 252Cdur% 252Cei% 252Cgir% 252Cid% 252C2c2% 252C2C2C2Ci2 252% 252C2C2Ci2 252C2Ci2 252C2CiCIT2 % 252Cmime% 252Cmm% 252Cmn% 252Cms% 252Cmv% 252Cpl% 252Crequiressl% 252Csource% 252Cexpire% 26itag% 3D140% 26gir% 3Dyes% 26ip% 3D81.217.53.239% 26id% 3He-ALNxMeQYw4LLc1FAjxt4h795wKTdqJnzc_SBgzEJVBxR% 26c% 3DWEB% 26keepalive% 3Dyes% 26mm % 3D31% 252C29% 3Dsn% 26 ppm-35cxanpbo5a-8pxl% 252Csn-2gb7sn7r% 26ei% 3Du4IIW7DfC46rgAfrrpnICw% 26ms% 3Dau% 252Crdu% 26mt% 3D1527284230% 26v% 3Dm1616 3D16M 3D16% 3Dm
Just decrypt the URL and you get what you want.
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