Jesus is responding to the Jews who are demanding He demonstrate who He says He is. Jesus has already performed miracles before them but their unbelief blinds them and they demand more. Jesus knows they are already lost and will never be convinced. He defines what they must become to be a follower; a listener to His voice.
Listening to the voice of Jesus can be hard or it can be easy. It becomes hard for me when the screech and scream of this world overwhelms me. Being a typical bloke, I am single minded. If my focus is on a thing of this world then Jesus' voice doesn't get much attention. However, this trait can work for me. When my focus is on Jesus; in prayer, at Mass, in adoration, then I do hear Him.