The best way to do this is to isolate what is making the sound.
If it's the pump, then a quieter pump. Place it on a mouse pad to soften vibration, and put a length of flexible vinyl hose between the pump and the sump and between the pump and the return line. Then the vibration of the pump is isolated.
Second, if there is a splashing noise into the sump, try a Herbie overflow style or beananimal overflow style.
Minimize the distance water must fall by shortening bulkheads in the sump. The only bulkhead you really need is to keep the skimmer area a constant water height, others can vary.
Fans can be replaced with quieter fans such as computer fans.
Ballasts should be located in places less humid.
That should about do it. In my experience the two most noisy parts of the sump are the drain line comming in from the tank, and the pump vibrating against the sump wall or the return piping. If you quiet them down, then you shouldn't need to insulate the cabinet.
Incidentally, auto stereo shops carry sound dampening insulating foam for use in speaker boxes and car doors and trunks. You may check those places out if you must insulate the cabinet.
Aaron.
