what part of the over flow are you referring
the overflow up top into over flow box can be remedied with a 16oz soda bottle cut bottle in half using top half cut a notch so it fits over over flow tub the cap serves as a air valve it may need to be loosened a little but the noise will still be gone
submersing it over the down spout of the overflow this will stop the gurgling
now for the outlet of over flow down at the sump this is the simplest remedy I know but it works wonders , this also takes the place of the filter sock all you need is a pond style pre filter you may need to use a plastic zip tie to attach , this will not only muffle the sound of running water splashing it will also filter out any crud ,
the pre filter will need to be rinsed once a week, all together this is a $4. fix and works wonders
I marked the cut out on bottle since the camera didn't pick it up as bottle was clear
you may also need to trim a small section of the pre filter off the top about 1 inch to plug the bottom of the pre filter , slip pre filter over down tub and zip tie it on
this should help solve the noise issue and its not a eye sore either