1. put a standpipe in the drain so the water level in the overflow is higher - less height for the water to fall equals less noise. Alternately, insert a plexiglass pane at an angle, with the top touching just under the teeth of your overflow. Water will run down the face of the pane and make less noise (but not zero noise) than a true vertical waterfall. I've seen some people use a ziplock baggie in lieu of the plastic pane.
2. reduce flowrate - using your return pump for circulation means high noise. Consider closed loop or in-tank powerheads instead.
3. If the noise is from the drain itself (as opposed to the water falling into the overflow), try a durso or stockman standpipe. This also inherently adds a standpipe to your overflow (see #1).