The Alarm...
So, the couple that lived above the store had to be silent all the time, or else the alarm would go off? What if they had to move the piano or something? :/
shareThe alarm responded to vibration, not noise. I suppose, theoretically, they could get so loud that it set off the alarm. Even chiselling through the ceiling didn't set it off, only if Jo really smashed that hammer into it. My guess is that there's nothing that the couple would do to inflict that level of vibration into the store below. There was a large amount of concrete, the ceiling of the store itself, plus the under-flooring, flooring, and carpet between the couple and the store. Finally, the couple were older - so they wouldn't be tap dancing or anything - and the guy was a co-owner of the store - aware of the alarm - and all of this adds up to an unlikely possibility that the alarm would trip accidentally.
shareThe robbers didn't need to be silent, then -.-
shareThe robbers' silence was caused by several factors, only one of which is the alarm.
The reality is that the alarm wasn't so sensitive that normal, daily life of an elderly couple would set it off. However, the robbers don't have precise figures. They would, naturally, be extra cautious.
Furthermore, a lot of the vibrations would be absorbed by the carpet and flooring. The robbers removed all of this.
Consider also: it's not just the alarm that the robbers need to worry about. If they're talking, it's one more noise that might alert somebody to their presence. Granted, they're already banging with a chisel, but why add to that noise? Why provide neighbours with voices to wake up to and match? Why give the elderly couple a sense of the timbre of their voices? This would also let the couple know that all four robbers were in the next room. They might try to escape if they know for a fact that they aren't being observed.
Finally, getting to talking serves no purpose, but in fact breeds distraction. Tony spent the whole robbery tracking the time and monitoring the patrols and deliveries that he and Jo had cased on previous occasions. He can't get caught up in chit-chat. He might want to hear into the street, too. What if a couple of cops are having a conversation beneath the window. "I think something's off? Should we go up? All right; have a ring." That kind of thing. Tony might not pick that out either way, but if the other three are jibber-jabbering? Not a chance.
Silence is smarter, more professional, and gives every opportunity for success that they can get - and they needed it all.
Just so happened to make a really great, tense, and thoroughly entertaining scene, too! Of course, you can make a brilliant bank tunnelling scene with loads of racket, too. The Bank Job (with Jason Statham) has all the proof you need.
Nah, its a plot hole.
shareIt doesn't create a flaw in the plot, so it's categorically not a plot hole.
shareYou're just making excuses. And a plot hole is an inconsistency, not necessarily a flaw.
shareThieves are extra silent to avoid tripping an alarm The alarm is wired so it'll go off if you hit it harder than an old couple walking over it through carpeting, flooring, and (what looked like) several inches of concrete.
Look, man, I'm just not seeing the plot hole.
Then put your glasses on.
shareYou know, I've put up a bunch of counter-arguments and logical assertions, and if you have specific refutations of my points, that's fine, but I have yet to hear them. I'm not convinced by the arguments, "Nah, its a plot hole," or "[I'm] just making excuses."
Do you have ways to address the specific points I've made about character and circumstance? I'm open to refutation.