> The very earliest ability to discern and produce
> sounds at about 9 months is called babbling. The
> baby seems to be talking away--but the pseudo
> speaking seems just to be exploring articulatory
> capabilities. Over the next months, the child
> seems to go through "babbling drift," when the
> speech sounds of the child's speaking community
> tend to dominate, ending in the pronunciations of
> meaningful words.
Babies communicate from birth but we are very poor at understanding it and reciprocating it. There are numerous reasons we are so poor but that babies have little ability to modulate their voices or to understand the world around them are chief among them. We don't expect communication so we tend to overlook it. We don't understand their world so we don't know what to look for. While there are commonalities between baby communication there are some differences as well and they all involve gestures, body language, and signs. The formatting for this means of communicating and understanding is different than the formatting we learn as language and use to communicate which makes memory retrieval of anything from our own babyhoods very improbable.
We tend to assume that since babies aren't saying much and can't understand much language it follows they don't think much but this really isn't true. Much of their activity is geared toward maturation and modelling their world in their brains. By the time they start babbling they are starting to work on learning their parents' language. But babies are very adept at learning important word meanings long before this though quite often such word must be signed to achieve actual communication.
> Full mastery of those sounds may never happen, but
> having enough control to communicate effectively
> is mastered by age four.
I believe "communication" now means something entirely different than it did to cavebabies or pyramid building babies. We take years to master modern language and simply never notice that this language must rewrite the brain and the way we think. We can't even imagine that we are born with what was once a perfectly "normal", "natural", and effective language which we outgrew.
> Meanwhile, the ability to notice visual details
> and control eye movements is very advanced atan
> early age. Putting sounds and symbols together is
> normally possible as soon as speech is ready.
> There's no reason why those two abilities couldn't
> have developed and remain somewhat separate from
> each other.
Babies understand far more of their world than your words seem to imply. They will certainly let everyone know when they are uncomfortable and provide information to let people know what's needed to become comfortable. But they also enjoy games, exercise, and clowning. They love to explore so crawling is of paramount importance to the very young and marks an important milestone. Even before they crawl they like expeditions and seeing new things. Odors and tastes are most easily keyed to memory so these are very stimulating for them. When they get a little older tactile stimulation becomes more important.
The whole time they've got a great deal to say but we can't "hear" them because we speak a different language.