There are some options. I am not sure wether you have an external overflow box or internal. If you have an internal, you can buy a standpipe setup (Durso), this quiets the overflow significantly - i use in my
FW tank internal overflow. External overflow box you can place a sponge in it - just be sure to clean frequently. For my
sump i use a diffuser/sponge configuration on the end of the drain hose. As to the skimmer - you can build a simple enclosure out of foam (not styrofoam), also make sure if it is adjustable, that you have it adjusted properely - so that it is operating effeciently - models with needle wheels, etc, can be noisier if not adjusted properely.
Hope this helps.