WASHINGTON — When Mark McKenzie first saw a synthesizer that could replicate the sound of an orchestra with one piece of equipment, the budding composer had a moment of gratitude. “I remember going, ‘Holy smokes, I’m sure glad I’m not a musician,’ ” McKenzie said. “ ‘I’m a composer, nobody’s going to replace me.’ ” An Oscar, four decades of credits and 125 movies later, machines are coming for his job. Studios and producers are always looking fo…