Thanks to modern science, we now understand that hearing actually happens in the brain, not the ears. The ears collect vibrations in the air and turn them into electrical signals which are sent via nerves to the brain; the brain sorts and interprets the signals and conveys them, with an interpretation of their meaning, to the conscious mind. Of course, this all happens instantaneously and without our awareness; we just hear sounds, voices, language, car horns, train whistles, chirping crickets, whatever. Our brains also do a marvelous job of sorting the sounds and assigning value: A new mother who lives near train tracks may barely hear the train whistle, but is instantly alert at her baby's quietest whimper.
It makes sense, then that “hearing” in the Bible means more than just the collection of atmospheric vibrations: It represents receiving and understanding new ideas, particularly those from the Lord. And on a deeper level it represents obedience, because we must understand to obey. This also is reflected in natural language, in which we often use “hear” to mean “obey.”
There are many shades of meaning beyond that, depending on context - who is hearing whom and what the subject at hand is.