bullets move faster then sound. the bullet would strike before he heard it. This applies to superman as well. unless he SEES it, no, even Superman wouldn't hear it being fired or anything.
Of course, Superman can hear in space somehow... so, at some point the physics of sound is lost..
This also applies to "sonics" being used as weapons vs both. superman is "weak" to sonics, because of his super hearing... but, hes also faster then sound.. so, uh.. which is it?
Already been over the stopping for no reason.
Biggest issue here is the "feel a bullet hit is skin, and react to it, before it pierces his skin" is some next level speed force level shit. up till now he's at like 600mph speed force/quicksilver stuff. THAT however, is like 50,000 mph or more. a 9mm will be like 700-900mph. the speed required to make a bullet traveling 800mph effectively stop is insane.
So, its normal Flash behavior. But, in the context of the show, its a massive power buff, no one seems to have noticed.