Instructions
Click on the sound icon on your tool bar.
Click "Mixer" if the option is available. This will give you refined control over which programs make noise. If there is no "Mixer" option, click on the "Mute" button to mute all sound on your computer.
Push the "Mute" button immediately beneath "Plugin Container for" the browser your "Moshi Monsters" program is in if you do have the "Mixer" option.