16mm film is silent if the sprocket holes are on both sides of the film. For sound, one side of the film’s sprocket holes are replaced by the soundtrack. The sound track could be magnetic or optical. Magnetic (or Co-mag film) is generally found on film used in news gathering and this track was recorded as the film was being shot. Optical sound track is created when the film is put together (sound is recorded separately from the film media) for creating a print and this is typically found on films that were sold or distributed.