Alex,
There is a split between the need to keep quiet and the need to maintain a cohesive social unit (which is done primarily with vocalization in our species.)
At face value, a child who is lost and alone in a dangerous environment must choose which behavior to perform. To cry for help from its group. Or to maintain silence so as not to attract predators.
This very case was played out recently in the case of the
missing cub scout from Utah.
Here is a boy who would live up to your expectations of being quiet. He obeyed his parents commands. Both to stay on the trail. And to not talk to strangers. Every time that a search party would pass by, the boy would hide!
But, no matter how quiet he was, I bet he wasn't anywhere near quiet enough to prevent notice from any hungry predator that might have been passing by.
The human animal is different than all other animals. We do not rely on the same survival mechanisms. We are no longer capable of the same stealth which is second nature to most animals. Not only this, but sound is only one avenue of discovery and quite possibly not the primary one.
Try covering up the scent of fear.
If that boy had not been found, then he would have died. Pure and simple.
His survival depended upon being found by his group. By humans. To be brought back into his social support group. If he hadn't been found, he would have been dead. Predated or not.
It is this that is at the heart of man's noisy nature. With our noise we proclaim territory. We let the animals know that MAN is here. That this is our woods. Our savannah. Our world. Step aside, Lion, or face the wrath of man.
Our noise is our territorial pissings.
I've observed this directly in certain types of people who are very territorial. They always say everything as loudly as they can so that as many people as possible can hear every word they say. And they don't just walk. They STOMP. These people unconsciously put emphasis on every word, action, and deed to make themselves known. Their steps shake the ground. Their words shake the air. (Their minds are usually small...)
This is what it means to live among a species whose chief means of communication lies in vocalizations.
Every heard a quiet monkey troop?
What do they do when they see a snake? Hush up? No. They go ape-shit.
What do they do each time any scuffle takes place?
Vocalization is obviously not a detriment to survival. Else, as you have said, we wouldn't be here.
So. When there appears to be a contradiction. Check your premises.
Your premise is that silence is more beneficial than noise.
Your premise is wrong.
Edit:
This is of course a tricky situation. As man is a social animal, his default behaviors have a tendency towards this. Hence we're noisy as this is the way for a troop to act. Also, when separated involuntarily from our group, we make noise to be reunited. But, when alone and there is no choice of being reunited with the group, then we must alter our ways. But, we must do so consciously as our subconscious instincts are largely social. We must concentrate upon remaining silent. We must concentrate on staying small.
Keep in mind that different animals have different criteria for judgement of prey. Some animals you are encouraged to puff yourself up. Make yourself big. Make lots of noise. Look them directly in the eyes.
Other animals you want to do just the opposite as the above behavior will drive them nuts and prompt them to kill you.
These are things that can only be learned through observation and are therefore in the conscious as opposed to the subconscious realm of action and behavior. They must be taught. Man has spread out across an entire planet and if we were hardwired to act in any specific way as if we were only to face specific predators, then we would never have spread the way we have.