It works okay in Internet Explorer?
From
Video or audio does not play - MozillaZine Knowledge Base
* Some sites may require you to allow popup windows. Allow the site as an exception in your popup blocker settings.
* Try clearing the cache.
* Some sites may require you to allow cookies or that you disable ad-blocking [2] so try the following:
o Make sure that cookies are enabled in your privacy settings and that the site is not being blocked in the exceptions list. Also try removing cookies for the problem sites to allow new cookies to be set.
o Temporarily disable any adblocking software or extensions such as Adblock or Adblock Plus. If the site works with ad-blocking disabled you may want to "whitelist" that specific site in your adblocking filter.
o If you use a Windows hosts file you can temporarily disable it by renaming it Xhosts, or you can edit the hosts file to remove specific entries such as ad.doubleclick.net that can cause videos on certain sites not to work [3]. You may need to flush the DNS cache [4] ("Start -> Run -> ipconfig /flushdns") for the hosts file change to take effect.
* Windows Vista has a per-application Volume Mixer that sometimes mutes or turns down the volume in Firefox or another application. [5] Go to a site that should have sound (see Testing plugins). With your Mozilla application still open, click the speaker icon in the Windows task bar, click "Mixer" and make sure the sound isn't muted and that the volume slider isn't at the bottom. [6] [7]
Just started happening yesterday.
