If you use the GMail Notifier for Windows, you might like me got irritated of the alert sound you get, when an email arrives. Even when you choose the no-sound scheme in Windows. The work-around is to use an empty wave-file.
Open the sound-recorder, and save the empty file (or download this empty wav file). Go to sound settings in the windows control panel, and lookup “New Mail Notification” in the Sounds-tab (like on the picture shown right). Select the empty-wav as sound for this event, and press OK.