edit please see my next post
This is a known problem. It may be solved for now by using the older version of FlashPlayer, which may be obtainable from their site.

I will look up and re-read further info and post back in a few minutes.

(If you are reading the email I suggest you follow the email link to https://support.mozilla.org/en-US/questions/929459#answer-341531 this information is formatted to be read on the website)

Sorry I can not give an immediate solution.

There seem to have been a few problems with Flashplayer recently. The sound problem was thought to have been solved, but has been around for a while.

1. Please state what version of FlashPlayer you are currently using.
2. then update to the current version, and confirm what version that is

• note the Flash site will check this for you and provide any download you need
  with Flash enabled in Firefox navigate to
  http://www.adobe.com/software/flash/about/

As you have Win7 may I ask you to try using Internet Explorer and say whether or not that also suffers this problem?

Please also confirm which version of Firefox you are using?
(note a minor update could be released this week so are you on 13.0 )

• To check the version navigate to about:support
  Key in about support into the Firefox address bar

Related thread Youtube, Netflix, and other sites continue playing audio after the tab is closed

Version of Flash is 11.3.200.257 Version of Fox is 13.0.1

I.E. ver. 9 does work. All other Fox add on are up to date and current.