"a camera's running speed is locked to the digitally divided oscillations of a quartz crystal, with an accuracy of a few parts per million. The function of the pilot cable was to feed a representation of the camera's speed to the recorder. Since with crystal control that speed is precisely known, the cable can be replaced by a similarly accurate crystal generator mounted in the audio recorder. This eliminates any connection between camera and recorder, but permits them to stay in sync with each other. A pilot signal is recorded as above, but it comes from the built-in crystal instead of the camera. The resulting tape is resolved to mag film just the same as a pilot tape. You still need a clapper board for a start mark."

So, it doesn't actually record the sound, but it is capable of being sync'd to the recording of the sound.