In “Why isn’t there an audio equivalent of the laser?” (http://www.straightdope.com/classics/a2_114.html ), the technology mentioned on the following websites isn’t mentioned:
http://www.atcsd.com/tl_hss.html
http://www.holosonics.com/technology.html
Although these sound technologies undoubtly differ from lasers in important ways, I think they are worth a mention in response to the question “Why haven’t they found a way to concentrate sound waves…”