To mute Flash apps:

• there are various plugins/tray apps/etc that do it
  • FlashMute
  • Mute Flash, for Firefox
• however, all the plugins do is create an empty file called "msacm32.dll" in the browser's directory. Note: you must restart the browser after making this change for it to take effect.
  • to "unmute", remove "msacm32.dll", and restart your browser again