When God first called the great Prophet Samuel, Samuel did not immediately recognize the voice of God! Samuel had to get advice from his mentor, the priest Eli, on what do when he heard God's voice calling to him. Only after the third calling from God did Samuel answer. Why didn't Samuel recognize God's voice? Maybe it was because he was not actively searching for God and listening for His voice? Once Samuel did learn the voice of God, Samuel followed God the rest of his life.
Even the best of us Christians who are seeking God probably could not recognize the voice of God if He did speak to us. Why? It it because the noise of the world drowns out the quite still voice of God? It is because we don't have a relationship with God and therefore treat His voice as if it were the voice of a stranger?
Sheep have to learn the voice of their specific shepherd over all the other voices in the fields. Once they do, they will only respond to THEIR Shepherd, and them alone. Maybe as Christians we should do a better job of learning the voice of our Shepherd, Jesus the Christ? There are many voices in our world who claim to be shepherds or just voices who want to distract us from hearing the voice of God. We need to learn the voice of God, because His is the only voice that will lead us to safety, among a world full of wolves!