The act of hearing and interpreting sounds/words is much more difficult than most of us realize. Believe it or not, you don’t hear with your ears, you hear with your brain. The fact is, hearing, which is caused when sound waves reach the ear drum and are converted to neural activity, is just the first step in a complicated process of utilizing sound-based sensory input relayed to the cognitive system of the brain. 
In the brain, sound goes through a series of steps, the first of which includes listening or “selecting information with attention and effort.” This also includes filtering out non-useful noise – ambient sound in a noisy restaurant.
The next cognitive step is comprehension, or interpreting context and grammatical information. The final step requires that the brain construct an “image” of the signal, which it compares with stored “images” in our memory to come up with a match. It is this “match” that allows us to understand what is being said and/or recognizes sounds with important meanings including sirens, doorbells, and the familiar cry of a baby.   
It is important to point out that the brain hears what it wants to hear. Each listener brings to the hearing experience a lifetime of cognitive development that is an important factor in hearing accurately. Research has directly related cognitive developmental delays to hearing impairments. It is believed that the brain spends more time, energy and space focusing on the act of listening in the hearing impaired person. Therefore, because more effort is actively involved in the “hearing” process, cognitive development is often slowed.  Studies have also shown that diminished cognitive abilities may lead to other hearing and non-hearing deficiencies including limited hearing and interpretation of non-speech and speech sounds, loss of spatial hearing, selective attention issues, poor memory, lack of problem-solving skills, slowed speech communication, and diminished access to the environment and others.  The bottom line: detection of the sounds must come first, no other levels of comprehension can occur without it.