You won't react fast enough woth how the A 10 is supposed to fly. It flies low, the sound is actually not that loud until it would be to late as it pops up and unloads.As fire control for AA autocannons our eyes and ears are obsolete. That I will not disagree with.
As for spotting something like an A-10? I can hear an airliner on its landing approach when it's several miles away. I can also hear my daughters trying to sneak out no matter how quiet they try to be because I can hear them opening the garage door when I'm asleep.
My hearing sucks. I have tinnitus and two blown eardrums. Daddy will be waiting and wanting an explanation when they get back from whatever shenanigans they thought they could get away with.
If you want to make something like an A-10 stealthy make it sound like background noise and paint it to look like the background.
No stealth aircraft is silent