Cognitive Daily have a very interesting post with examples of what babies hear. There are two samples of a baby’s name being called over background noise and the only difference between them is that the baby’s name is 5db louder in the first clip. In a study on babies they found that this 5db difference made a significant impact on whether the baby responded to their name being called.
In adults, the inability to hear properly when there is background noise is known as Cocktail Party Deafness. It has been linked to the sort of hearing deficits that found in dyslexics and ADHD. It is hard not to speculate that this sort of hearing problem is related to an underdeveloped or immature brain as many symptoms of dyslexia and ADHD are also indicative of underdevelopment.