In order to make the overflows run as quietly as possible, I used the following design for both overflow boxes.
Each overflow box has two 1.5" outlets. The primary outlet has a gate valve which chokes off the flow until the water starts to rise in the overflow box. In this way, the primary essentially siphons water out of the overflow box. As no air is drawn into the siphon it is silent. The primary outlet water is released into the sump below the water line resulting in no splashing or bubbling.
The secondary outlet has a stand pipe that is higher than the water level in the overflow box. This outlet is not in use as the primary carries away all of the volume. Should the primary outlet become blocked, the water level in the overflow box will rise until the secondary outlet becomes active. I can simulate a worst case scenario by shutting the primary gate valve off completely. At that point, the secondary outlet carries 100% of the flow and does so with typical noise!
Using this system, the ONLY sound is the sound of the water falling 1-2" inside the overflow box. I can probably muffle that by adding some sponge to the overflow box so the water has a softer landing.
This is a side view of one of the overflow boxes. You can see how the water level stays well above the primary oulet in the bottom, but short of the secondary outlet standtube in the background. What little noise there is was caused by some of those bubbles you see getting sucked down into the primary. This has since been corrected by adding a layer of foam at the surface above the primary outlet.
