Legend has it that when "Talkies" were first replacing the silent films, this expression originated from Director Joseph Von Sternberg, who often did not want to record sound during a scene, apparently so that he could "yell at" his Actors and tell them how to perform while filming.  So he would frequently shout out with his thick German accent "We will film this scene Mit Out Sound!" ...and to this day, M.O.S. means: "with out sound."