rekado gave us an important hint: to set the environment variables properly. This and installing the GStreamer plugins mentioned should be enough.
However, my experience tells me that it's better to visit YouTube with GNU Icecat + NoScript + GreaseMonkey + ViewTube. You'll avoid running non-free software that would otherwise be forced to you automatically through JavaScript. ViewTube supports getting videos that youtube-dl seems to be struggling to get. Also, if I'm not mistaken there's a *possible* (not confirmed) non-free software dependency in youtube-dl (and also mpv due to its usage of youtube-dl), this was reported to the gnu-linux-libre mailing list (the workgroup for free/libre system distributions, not the GNU Linux-libre project), see [[http://lists.nongnu.org/archive/html/gnu-linux-libre/2017-04/msg00001.html]]. NOTE I do support/like and use GNU LibreJS for most other websites (I even offer basic consulting and training on how to make pages according to GNU LibreJS), but it seems that GNU LibreJS is currently blocking GreaseMonkey userscripts (including ViewTube), this didn't seem to happen in previous versions. I still didn't test the recent GNU IceCat update, so I can't tell if the problem still exists. -- - [[https://libreplanet.org/wiki/User:Adfeno]] - Palestrante e consultor sobre /software/ livre (não confundir com gratis). - "WhatsApp"? Ele não é livre, por isso não uso. Iguais a ele prefiro GNU Ring, ou Tox. Quer outras formas de contato? Adicione o vCard que está no endereço acima aos teus contatos. - Pretende me enviar arquivos .doc, .ppt, .cdr, ou .mp3? OK, eu aceito, mas não repasso. Entrego apenas em formatos favoráveis ao /software/ livre. Favor entrar em contato em caso de dúvida.