Why is Jesus more concerned with how we hear, than who or what we hear? Because, whoever or whatever we hear we must hear in faith. Christ wants us to take care to hear in faith.
Little makes sense when heard without faith. Those who heard Christ with faithless ears did not hear the truth he spoke, the mercy he preached, or the charity he taught. Those who hear with ears of the world, ears that lack faith, do not hear the truth that His Church teaches, or the mercy and forgiveness she preaches, or the love she speaks with. Take care how you hear. Take care to hear in faith.