Matthew 13:9-13 Who hath ears to hear, let him hear.  And the disciples came, and said to him, Why speakest thou to them in parables?  He answered and said to them, Because it is given to you to know the mysteries of the kingdom of heaven, but to them it is not given.  For whosoever hath, to him shall be given, and he shall have more abundance: but whosoever hath not, from him shall be taken away even that he hath.  Therefore speak I to them in parables: because they seeing see not; and hearing they hear not, neither do they understand.

Not everyone hears God's voice.  Not everyone can understand.