Is one pipe your return? If so then you will need to get a longer drain pipe to set the water level. There are various ways to quiet the drain, to many to go into detail here. By raising the height of your drain pipe you will stop the noise of the water falling into the overflow. Google gurgle buster, or overflow silencer.
The reason I ask if 1 is your return is there is another way to use the two hole overflow to help quiet it down more you will need to return your tank water over the back of the tank though. Its called a herbie style, google that. Its kind of what I'm doing on my setup. One pipe has a valve and is shorter then the other both are used to drain the water one is the emergency the one with the valve is the main drain and is submergerged the valve is opened or closed to keep water above the pipes opening, it runs pretty silent but it seems to me that you have to tweak the valve occasionally to keep it dialed in. This is also called a syphon overflow. Do some googling and you will see.