Possibly a coincidence. Some keyboards may include a mute button, but I would only expect that to do the equivalent of toggling the mute setting. Where does the sound normally come from ? if an external speaker set are they connected properly still or did they come loose when cleaned. This is not particularly a firefox problem, unless you only loose sound when browsing. Windows Vista probably has troubleshooting advice available or use firefox to look at http://windows.microsoft.com/en-us/windows/help/no-sound-in-windows