I have Intel HDA (dont know if happens to other brands)
I had this problem and I though it was an outdated flash package or something. It turns out that when I check on the volume in the taskbar and enter mixer PCM was muted. If you raise it all the videos will be heard
Though it would help others with similar problems at least to diagnose it because I almost burned my brain thinking and doing lot of sht and it turned out it was something as simple like that.
Thanks for your suggestion El_Cangri on checking your mixer configuration to make sure it is setup properly. I might also add you may want to look at the channels option as not all sound controls may be added and perhaps one or more need to be added. Your advice to taking a closer look at your mixer settings is very ***Sound ***advice. lol!
In my case, the volume level of the PCM was in the middle, I mean, it was’nt muted at all, and just by taking to the max level of the PCM device I could finally hear sound on youtube. So, try by taking to the max level the PCM.