I just started working with hydro and it would appear as though the loudest thing by far is the pump for the air stones. Now I have thought about downgrading to like 3 smaller ones but I would hate to lose that kind of air flow if there was an easy way to control the volume. I grow in a garage and don't mind if you can hear it in the garage, it's just a matter of people being able to hear it maybe through the garage door. I don't know if there is any basic or even advanced stuff for sound control. This thing is tricky because it is just made to vibrate.
In case it matters. This is the air pump http://www.amazon.com/gp/product/B002JPRNOU/ref=oh_aui_detailpage_o07_s01?ie=UTF8&psc=1