The sounds of silence will soon reign supreme in Boston’s subway tunnels and station stops. Well, except for the screeching trains and squawking public address systems, of course. The city has banned street musicians from using amplification or playing electronic keyboards or brass instruments on Boston’s underground platforms. “The rules are sure to transform Boston’s true underground music scene, which up to this point has been one of the nation’s least regulated. In New York and Atlanta, musicians must audition and sign up for slots; Toronto singers pay a $114 fee; in London, musicians need licenses to croon to commuters ‘minding the gap.’ And in Washington, D.C., they’re banned altogether.”