No sound on YouTube

[Teresa e Junior]

On a fresh install of Xubuntu 16.04.1, the sound works OK on all 
applications I have tried, and also works well with Flash and HTML5 
audio on the test page http://hpr.dogphilosophy.net/test/.

But the problem is that YouTube videos with HTML5 don't output any 
sound, either in Google Chrome and Firefox. The stream appears on 
pavucontrol, and I have tried changing both its volume and output 
device, without any success. PulseAudio is version 1:8.0-0ubuntu3.