In a voice that was at first inaudible, Senator Edward M. Kennedy pleaded guilty this morning in district court to a charge of leaving the scene of an accident after knowingly causing injury to Miss Mary Jo Kopechne without making himself known. Judge James A. Boyle imposed a suspended two-month sentence to the house of correction at Barnstable, with the comment that “the defendant has already been and will continue to be punished far beyond anything this court can impose.”