No sound for Youtube in Firefox but Opera browser plays the Youtube

There are a lot of bad or obvious tips on how to cure You tube sound dropouts.

I have one machine on a homenet that plays Firefox You tube no problem.
On the second node, pretty much all the same components but You tube
will not play audio in Firefox browser. I switch to load the You tube on Opera browser and it plays.

Can this be in the browser? No new hardware so the tip to update drivers
does not apply. Again, this is not where I zeroed the sound on the You tube screen or muted something else. The stuff plays in Opera.

Any clues on an answer to it?