Getting my sound back by closing tab, undo close tab

I keep losing my sound under Ubuntu 14.04 and firefox trying to play youtube with an Optiplex 760.

For some reason I hear no sound at all, but what seems to work is to close the youtube tab, then right click on the tabs area and click on "Undo close tab". Most of the time the sound will magically start working.

Reloading the page doesn't seem to work, I have to fully close the tab.

It'd be nice if I could do that as a single menu item, "Close tab and reopen", or have the most recently closed tabs listed on the new tab page. I was going up to the menus and choosing History/Recently closed tabs until I found the right click menu item "Undo close tab".