Make a box and inside it line the walls with sound proof styrofoam. I did this with my sump pump, it is located in my playroom beside a spare bedroom so I had to keep it quiet. And it works. I made the box in finished plywood, stained and varnished, added a lamp on it and now looks like a piece of furniture.
at the risk of offending lazer's home decorating talents, submersible sump pump's aren't particularly noisy,,, is yours a pedestal pump ? the bearings may be getting ready to take a dump,,, the sump should have a cover installed so just glue soundproofing mtl to the underside,,, you COULD even place a floorlamp on it when done .