Starting the tutorial and want to turn down the sound of the factory. Been to options and put it down to 20%, still the same. Appears the factory volume slider has no effect.
Be sure to hit the “Apply changes” button at the top of the options menu. For me, any (or most?) changes to the options were ignored until I hit that button.
That is correct - options are only saved if you hit “Apply Changes”.
I know that. So I double checked, makes no difference, still the volume for the factory sound appears to stay the same level despite showing it lower.
EDIT-
Correction. Just tried again. It’s not the main factory noise it appears to be the hum. There is a hum that plays, it appears to be that hum where the volume never changes, the hum is the same volume even with all options on 0. As I play a video on another monitor, sometimes loud, it then means the hum is also loud. I then have to use Windows volume settings to turn down the volume of Big Phama.
So appears to simply need a volume slider for the hum 
I can confirm gi joe’s findings. It’s the background white noise that you get. To reproduce, start up a game, turn factory, gui, and music volumes to zero, and then crank up your computer’s volume. That white noise should be tied to the factory volume slider, but it’s not.