Q: Is there evidence that a fetus would differentiate between, say, a ticking clock and the sound of a mother saying something sweet?
A: If the question is simply, "can the fetus tell the difference between a ticking clock and its mothers voice," the answer is "definitely." They seem to be able to learn to distinguish between many sounds. If the question is "do they prefer" the mother saying something sweet, here we have only weak evidence suggesting that human voices, and in particular, sounds that are soothing and calming, are beneficial.
Q: Can scientists do brain scans shortly after a child¹s birth to see if his or her language skills are above normal, or below?
A: This is the Holy Grail of infancy language research (or at least, one of them): How can you tell if an infant WILL develop normal (or above normal, or below normal) language, since children don't generally speak until their second year of life? It's like asking "Will my 1-year-old be a world champion chess player/acrobat/artist…or will they fail math in 8th grade?" Since we can't tell directly, we have to look for predictors ("markers"), and these are generally only weak risk factors.
One of the main practical goals of infancy research now is precisely the search for such predictors. Why would we want to have such predictors? It's because we know that the human brain is especially flexible (the word "plastic" is often used as well) during the first few years of life. This is a period where the brain itself is developing at a breakneck speed. It's also a time where, if we believe there is brain damage, or even just a mild learning disability, the effect of intervening will almost certainly be greater than if we wait until later in life. One of the most dramatic illustrations of such flexibility is what happens when you have a stroke. An adult who has a stroke in the left part of the brain will (depending on the exact region) probably develop a language disorder. Depending also on the severity of the stroke, he may or may not recover significantly. But if a fetus or newly born infant has a stroke in the same region, he will almost certainly recover normal or almost normal language function. That is not because the damaged brain tissue grows back, but because other areas of the brain get recruited to help do the job that the damaged brain tissue can no longer do.
Q: What about the first year or two after birth? Can you tell anything about how a child will develop by the amount and type of sounds and words a child hears?
A: At this point, we know something very simple but important: There is a very high correlation between the amount of language input that a child hears and (a) how quickly they will be able to learn new words, and how quickly they will understand language, and (b) this beneficial effect of input (more is better) seems to have effects that last at least until second or third grade and almost certainly beyond. (We don't know how long because the studies looking at this have just started.) What we do not yet know is whether some types of language input are better than others, though we do believe that it's not enough to simply overhear language. For example, the child who hears 10,000 words a day that are spoken directly to him/her will benefit much more than the child who listens to his or her parent talking on their cell phone all day saying the same number of words.