You know, I don't really know of anything on the urban design scale, but I have dealt with soundscaping issues within the confines of my apartment. I am home a great deal and I homeschool my kids, so they are here "all the time". Acoustics in this apartment were just terrible when we first moved in and we had a lot of propblems with "volume wars" between the TV and the computers, for example. And my oldest son has sensory issues and he gets to be extremely difficult to deal with when he is experiencing sensory overload. So, in reading through this thread, it seems to me that folks are focusing on "sounds barriers" and missing the materials aspect (I know cololi mentioned it in terms of restaurant acoustic, but it seems to have largely been overlooked).
The reason a vegetated berm would be superior to "sound walls" is because it absorbs the sound rather than trying to "contain" it. Sound walls of bare stone strike me as an attempt to 'bottle it up'. But bare stone is an acoustic problem, similar to the metal ceiling of a restaurant. Think of how sound echoes harshly in a canyon with bare rock walls. Why does it do that? Well, partly due to the bare rock walls. (I am no expert in acoustics, but that is how I understand it.)
While water is useful for creating soothing sounds, like vegetation, it also has different acoustic properties from stone or brick walls. So, it does more than just mask noise. Just as soft furnishings, carpets, and curtains dramatically reduce unpleasant noise in interior design, "softscape" materials (like vegetation, an earth berm rather than a stone wall, and water features) shoul positively impact the public soundscape by altering the acoustics of the environment.
In addition to that, a "baffle" which causes noise to have to travel around corners and causes it to be reduced with each bounce (rather than amplified) can be really useful. But the baffle itself must be made of sound-reducing or sound-absorbing material -- and stone or masonry walls don't strike me as an effective baffle because they are made of the wrong kind of material, acoustically speaking.
As an example of an effective baffle on a small scale: My oldest son, who has sensory issues and for whom noise is such a problem, spends most of his day in the same room where I have my office set up -- and he listens incessantly to WAV files. He was making me nuts for a time and his desk area had terrible acoustics. We repositioned his speakers but it didn't really help. The thing that did help was to switch out the hard-surface folding screen behind his desk for an upholstered folding screen that happens to also be taller. That completely changed the acoustics of the area and made his "office" more pleasant for him to be in and made my life a lot less miserable.
It does not attempt to "contain" the noise. But it does dramatically reduce how noisy the place is and it changes the quality of the sound.
Selection of building materials, placement and relationship of buildings, and landscaping should all have a roll in soundscaping an urban environment. Or so I should think.
