Comment 1 for bug 1015803

Found the problem. It's an odd one. Looking in the Volume Control app (not sure if it's a Gnome or Xfce thing - desktop is Xfce), it showed that it has been defaulting to trying to play the audio stream through "Redwood HMDI Audio" on account of seeing a Radeon video card in the system. Turning that off under the Configuration tab and having Built-in Audio set to the appropriate (in my case) Analog Stereo Audio not only restores sound, but it fixes the Flash-video-racing-ahead problem.

What's insane is that video should race ahead because of a non-functioning audio output. How could Adobe have ever programmed Flash with that dependency?

In any case it appears that in the upgrade cycle Ubuntu insisted on bringing HDMI audio back into play. That had been set correctly - off - before the upgrade. I do have HDMI to my monitor, but its speakers are analog.