I'm not sure if I answered this in full or part once before or not...I think you have some similar threads going about drain and sump issues.
Overflow water level fluctuating
Reef Ready Corner Overflow Noise
So forgive me if any of this is a repeat.
Check that you aren't running too much flow first. If you are, you may be able to prevent the noise rather than trying to contain it.
Generally 2x to 4x the display's volume in GPH - actual flow, not a pump's rated flow - is all it takes to keep your skimmers and such well-fed. More can cause issues with bubbles and noise.
For example, consider a 200 gallon tank.
200 gallons
x2
=400 GPH
x4 = 800 GPH
You have to account for head loss if you're calculating your flow, but if at all possible, I like to actually measure the flow myself. Use a gallon jug on your return and time how long it takes to fill up....convert to GPH.
If you're flow rate is close to the correct range, then slowing down the flow as it hits the sump and then containing the bubbles will be your tasks. Enlarging the plumbing can help, as can letting the drain go into an intermediate container before being released into the sump. There are lots of ways that do variously good jobs of containing or limiting this problem.
Check your flow first to see if you can avoid having the problem in the first place though!
