I'm sorry but I don't understand some of your terms.

The overflow box has a drain, which drains into the sump.

The return pump pushes water back up into the tank via return lines.

There is NO WAY your tank can drain faster than your pump can push water back up, because they reach equilibrium within a couple of minutes. If the pump pushes 10g of 'new' water into the tank, there is now 10g worth of water that can drain down again. Here's my page about sumps, in case it helps:
http://www.melevsreef.com/what_sump.html

What return pump do you have now?

There are ways to quiet down a drain line, and a way to quiet down the area where water pours into the sump.