how can I turn off that default menu pop up sound

My brothers trying to sleep and doesn't mind me watching tv or chatiing in my room but he somehow can hear that sound that this game makes when you open an IM or any other window and he can't get to sleep when I have the sound up loud enough to hear it.
does anyone know how to change that sound or turn off just that sound it's a low thud and carries well I guess.

RACERX! CRAZY RABBIT DEWD!

Here's how... hippity hop till ya don't stop...

Go to PREFERENCES

AUDIO/VIDEO tab on tha left

See the UI Volume slider? Slide that ALL the way to the left and save settings.

THAT WILL DO THE TRICK! *does spazzorama*

Thanks