Sound from every window

Occasionally I am getting sound from every window whilst playing wow. This is annoying because I am hearing in game music, menu button effects etc like there is an echo.

This does not always happen. Usually just the one window has the sound.

Anybody know what might be causing this? And how I can prevent this from happening?

[Alge]

If you have your file virtualisation set up correctly and this occurs, then presumably you are pressing a keybinding that turns sound on or off. Check your keybindings in WoW to see what that is and then look for mapped keys sending that combination of keystrokes to the game.