If the voice of the shepherd is the word of God, then what is the voice of strangers?

John 10:3 To him the porter openeth; and the sheep hear his voice: and he calleth his own sheep by name, and leadeth them out.

  4 And when he putteth forth his own sheep, he goeth before them, and the sheep follow him: for they know his voice.

  5 And a stranger will they not follow, but will flee from him: for they know not the voice of strangers.

What is that other voice?

Where is the voice of strangers heard?  tv?  youtube?