IT TOOK A TRAGEDY for everyone to notice. In March 1995 Selena was murdered, and suddenly Americawhich is to say the Anglo mediadiscovered a new, no longer silent minority. If Hispanics were willing to buy millions of records by their own Madonna, what else might they consume? The raw numbers were tantalizing: The 1990 census, which was expected to count 20 million Hispanics, uncovered 23 million. And by the year 2010, Hispanics will be America's largest minority group.