What effect would putting in a large berm (maybe six feet high) for noise reduction and landscape interest have on the mature trees nearby?

HI Megan, We do not recommend adding soil much more that a few inches around trees. Added soil changes the dynamics of air and water that the roots are accustomed. However it is less risky depending on the tree species (silver maple tolerates it better than white oak for instance) and also depends on how much root area the berm impacts. such as if it is just one quarter of the drip line as opposed to 2/3. hope this helps in your decision. Sandy