I think the only thing that will shut up a high flow sump system is:
-3/4" (or even 1" for that matter) solid wood frame with felt and rubber door stops
-Insulated inside of the cabinet (carpet or foam)
-Hanging / cushioned (suspended in water) water pumps
-Soft flex tubing insulated for sound by pool noodles
-Enclosed wooded return run for the tubing to run back up to the canopy in to insulate and keep the return lines quiet in the cabinet
-No hard contacts to the wood cabinet anywhere, cushion it all
-VERY Low noise fans in canopy
-Oversized plumping to avoid "suction" opportunity sounds
-Standpipe method for overflow
-High water level in the overflow so as not to waterfall into corner box
-Minimal sharp bends if you're using hard plumbing (use 45s, not 90s)
-Step up the size of the tubing in the canopy to drop the pressure and reduce flow noise at spray bar
-Silence the canopy with foam or felt stoppers where it lifts open
-Use a ball valve at the bottom of the drop down to the sump to control the water level in the drain line to eliminate "air suction" from the bottom
If you're not doing all those things, it a high flow sump system just WON'T be quite!
Just kidding!
I haven't done ALL these things (most I have though) and my 90g FW sump system with 500gph sump is still loud. Hope those are some ideas however to help you shut up your system! I've given up, the pump itself is my loudest part of the system, which echos worst in the canopy due to some horrible resonate frequencies in the canopy cabinet...
Have fun!!