Finding whats causing youtbe to remain silent is a bit tricky with all the new codecs. The best solution in firefox FF65 is to turnoff or disable adobe. Youtube doesn't use Adobe flash any more. it's published on their website. They use HTML5 as their default.
Open Synapetic package manager and enable (mark) the following packages for install on mint19.1 to ensure videos will play in any format:
- Firefox web driver ~python.
- 'Firefoxdriver' ~metadata codec
- lib5 ~html5 java plugin for jplayer
disable: codecs for pepper & naci (mark for removal)
If your using chromium then you only need to enable "ffmpeg-extras' it should install by default.
If your still having trouble try downloading the video and playing it with VLC-player.