The fact that someone took the sound out indicates to me that the (probably male) PBS narrator's voice interfered with their...er... enjoyment... of really big industrial machines.
what is completely astounding to me is, that at one point someone saw traditional dump trucks and backhoes etc, and thought, this is not fast enough. There has to be a way to take portions of the mountain away at once... and there was. they designed the most ridiculous and most bad ass machine ever.
It's like the kind of thing you'd see in some dystopian future sci fi thing, like rusting and skeletal in the background as Thundarr and Ooklah and Ariel ride by.
You may be interested to know that if the audio were there, it would tell us that this thing needs to eat four times its weight in dirt a day, and so it evolved that rotary mouth.