Xubuntu 14.04.1: No sound from Firefox 31.0

[Jeffery Small]

I have a new installation of Xubuntu 14.04.1 on a Xeon system.  Audio is
working fine with all music players as well as with the Opera and Chrome
browsers.  However, I cannot get Firefox 31.0 to play any sound.  For
example, a song from a music site or a youtube video play, but there is
no sound.

I have uninstalled/reinstalled the Firefox package and added the (probably
unnecessary) gecko-mediaplayer plugin without any change.  Here is a link
to a screenshot showing the active plugins.

    http://smallthoughts.com/photos/misc/gecko_plugin.png

I tried setting all the plugins to "Always Active" rather than "Ask to
Activate", I created a new empty user profile, and I have run firefox in
safe mode.  None of these attempts produced any sound.

Does anyone have any suggestions as to what might be wrong?  Any ideas
would be appreciated.  I've got to get this darn thing working!

Thanks.