In my very humble opinion, the better players learn over time, after repeated exposure to events, what to do and what not to do. Will they still make mistakes? Absolutely.
In my own son's case, it took years of talking about not allowing ANY outside distractions to affect him at the plate. His routine is to keep one foot in the box while he gets the sign. After he steps fully into the box he hears as close to nothing as possible. And count dad as one of those things he's learned not to hear.
I think that the greater issue for a player then noise, is expectation. Are you "expecting" a hit. I'm one of those that believes that those kinds of thoughts can not be allowed to manifest themselves in your brain while at the plate. While my son has faired well with that, I've seen more issues with that then crowd noise during his time.
In the few events where he has played with a couple of hundred fans or more, noise was never an issue for him. And fortunately he's done well not allowing outside thoughts during those events.
But a player has to experience it, fail at it, and learn from it. Another reason why baseball is such a difficult game.