Where is the sound coming from, the crashing of the water in the sump, or up top where the water is coming into the prefilter box? If it's up top, you can raise the height of the standpipe to make less distance for the water to fall. This should quiet it down a bit. If it's the mixture of air and water going into the pipe that's noisy, you might try shoving a piece of airline tubing down the drain pipe until it quiets, then taping it off somewhere.
Down below you may need to implement filter socks(I hope you have these already, for the large particulate matter) and possibly tilt the pipes so the water comes down on an angle or maybe put a T on them.