There really aren't any trees that are "sound proof". What I would suggest is a Cedar hedge, but to increase the sound proofing, consider building up a berm of soil before planting, especialy if it is direct noise (car traffic).
Build a soil berm as high as possible, then plant two offset rows of Cedar hedging. It may take some time to fill in, but this may be your best bet.

Maybe look into the possibility of bamboo! I know it has a noise reducing quality and it provides a nice rustling effect that some find relaxing while others find annoying. Just beware of how invasive some types can be.