Andy and Red have a conversation about this when Andy first asks Red to acquire the item. The shear size of the item leads one to believe that it would take forever to dig a route out. Plus, neither the guard nor the warden know the item exists because Andy used his bible to hide it.
There are many things to consider for this one. For one, the walls were pretty thick when you consider how long it takes him to crawl through his hand-carved hole. So that could muffle the sound from that angle. As far as the thunder...I'm sure we've all been in one of those storms that is just so loud you can't hear yourself think. That could've been one of those.
He probably carved that chunk out while digging his hole. Since he claimed to be a rockhound, he knew what kind of rock it was, how thick it would have to be, etc.
Supposing since he worked in the Warden's office, he probably saw blueprints to the complex, and saw where pipes ran.
Given the time of the matter, and the fact that the guards weren't really vigil to begin with, or he wouldn't have had a giant hole in the wall, I'm guessing he was probably betting on the fact they wouldn't discover him gone until the next morning.
Now, while I'm not trying to debunk all your reasons for why you feel that Shawshank is less superior to The Green Mile, I think if you give Shawshank repeated viewings you'll catch subtle things you might miss the first or second time around.
Spudracer, you use your imagination to come up with several possibilities to help explain the circumstances but it's pure supposition.
What I mean is, you're doing all the work to explain how Andy made his escape, while the playwright and director provide no backup information to support those points.
For instance, "Supposing since he worked in the Warden's office, he probably saw blueprints to the complex, and saw where pipes ran." Do wardens normally have in their pocession blueprints of their prisons, and if so, would those blueprints simply be stuck in a drawer accessable to trustees? Does your employer have a blueprint of the building where you work? Do you have a blueprint of the house where you live? Sure, a blueprint would explain how Andy had knowledge of the prison sewer system, but in real life relatively few people have interest in or access to blueprints of buildings, especially buildings in which they are incarcerated.
As for Andy being a rockhound, for all I know, he may have been a trained geologist who just happened to be working as a banker-bookkeeper. But I don't think knowledge of rocks carries over to man-made material such as concrete, which would be more in the line of a construction engineer. Even if we accept that Andy knows as much about rocks and concrete as he does about bookeeping, how big a hunk of concrete could he repeatedly raise above his head and smash down on the iron pipe he's straddling? 50 lbs, 25 lbs, less? And if you bang iron and concrete together, which is more likely to break first? To quote Sancho in
Man of La Mancha, "It doesn't matter if the pitcher hits the rock or the rock hits the pitcher, it's going to be bad for the pitcher," or in this case, the concrete.
We do agree that the prison guards were less than vigilant! But even so, how much lead time does Andy have when he makes his escape? I don't know the schedule at prisons, but when I was in the Army back in the '60s, it was lights out in the barracks about 10 p.m. and reville around 6-6:30 a.m., sometimes earlier. If the prison ran on the same institutional time frame, Andy would probably wait an hour or two for everyone to go to sleep, then make his way through the tunnel, smash open the pipe, and wiggle his way to freedom. That would take at least a couple of hours, don't you think? I would guess more, but say he gets out of prison and washed off by 2 a.m., he's got at most 4.5 hours to put distance between himself and the prison. How many miles could Andy walk in an hour through a blinding rain? Three, maybe four? I mean, here's a guy who has been in prison and working behind a desk for years, so how fit is he? Say he can really make time, 5 mph on the level surface of a road. But would he take the road where he's easily spotted if a vehicle comes by? Or does he go cross country at a slower speed tracking through the mud? And does he wear his prison uniform or go naked? Somewhere along the way, he's got to get clothing of some kind and transportation. So at the very most, Andy can cover 22.5 miles before he's missed and an alarm is sounded. Standard procedure is to immediately shut off and search the area within a certain radius of the prison in hopes of finding the prisoner hiding or finding someone who has seen him or finding someone whose car or bicycle or other transport has been stolen. I just don't think Andy the bookeeper could have made such an escape. But it's just a movie, so the writer and director count on us to cut them all the slack they want, just so they can use the gimmick of putting a movie poster on the wall so the Warden can throw a rock through it.