Quietest from all skimmers, I tried, is Tunze Nano Doc Skimmer 9002: small footprint in-tank skimmer.
Review is here: link .
It is not dead silent, sound is distracting as a dull headache, but doesn't interfere with daily activities, unless you have to do some creative or analyzing work. Allows to sleep.
Still is the best, comparing to air driven and venturi skimmers (tried: Lee's CC, Visi-Jet, ASM Mini, Bak-Pak2, Rio Nano HOB).
Dendro, my tank parts may not apply to you because I don't have an all-in-one setup. I have a sump hidden away in a cabinet. Specs are in my sig. Note that many things, including my overflow and return, have the potential to generate alot of noise, so I had to improvise to get them quiet. My Ocean Runner 2500 is encased in a rubber housing (from Eheim but still works) and my overflow drain sits right above a small drain gully stuffed with poly filter. I also use 2 Koralia Nanos and those are silent in my tank.
I got mine new from EBay. There is also Tunze Nano Cleaner (you can see it through Google Image Search), similar box, placed next to skimmer. It works independently: surface skimmer, pump at the bottom and filter media could be placed all the way above the pump.
Planning to get it later.
Can you post details and maybe photos of silencing? I have other tanks too, and one with overflow box makes quite a sound by water, gurgling down the flexible tubing.
Have Ocean Runner 2500 PH, it is not as quiet, as I would want it to be. What is rubber housing?
You may make a separate thread, if you prefer.
I hope, others will be interested in quieting equipment too.