Hmmmm... When I was bothered by the sound my overflow made (though it did not compare to the sound my 2 year old makes when she's angry), I jut put a sponge like the kind that sits over the intake for the pump that exports the water back from the fuge up to the tank in the noisy compartment of the overflow box so the water doesn't fall as far. I don't hear anything now, and it serves as a filter to boot! Was basically free, too.
Wish it was that easy to quiet the 2 year old...
-Daisy