I've seen a few questions about sound waves and sound travel here, so I'm hoping this question is a good fit for this site.

In my research about soundproofing, I've seen many accoustics gurus say that, when soundproofing a wall, you have to account for every single little opening in a structure like a wall or ceiling even if the opening is as tiny as the shaft of a screw. If you don't, you may as well not soundproof the wall at all.

Being that I don't understand the physics sound waves, my incredulity brings me to doubt those words. I imagine sound waves hitting a wall being much like taking a handfull of pebbles and slinging them at the same wall. When they leave your hand, they spread out. If I were to sling a handful of pebbles at a group of people from a distance, the chances are high that they're all going to get hit. If you put up a solid wall in front of them, however, they won't feel a thing as the wall will stop 100% of the pebbles. If you cut a hole 2' in diameter and sling the pebbles, perhaps only a few people will get hit since only the pebbles that make it through the hole will get through. If I were to apply the "weakest link" argument here, it would suggest that slinging a handful of pebbles at a wall with a small hole in it would be the same thing as slinging pebbles as if the wall weren't there. The only way this could be true is if all the pebbles spread out after leaving your hand, magically coalesced enough to fit through the hole, then spread out again to hit the group of people.

Now back to sound travel. How could a small, screw-sized hole totally negate the entire soundproofing effort? While I can understand that a little sound would still make it through that hole, I am not seeing how that little leak would make it as if the wall isn't there at all.

Can someone explain how a small opening in a well-soundproofed wall affects sound travel?