On RejZoR-Sharp I don't get a ding but a muted sort of click sound when I hover on the buttons.
This is the same as I get on a number of skins, aVist by SCZ, Silhoette_4in1, Maclover, so it would appear that is on a lot of the skins. But I rarely hear it as I don't have my volume full on and I only use the speakers on my monitor.
When I check the 'Disable avast! sounds, they are completely muted, on winXP Pro.