Hearing the voice of God is highly subjective and is therefore difficult to teach or even to explain. It would be much easier if God would speak to us in an audible voice, although the problem then would be to understand how to apply what He said. In my own experience and in talking to others, I have concluded that there is no particular advantage in hearing God speak in an audible voice (except to the hearing impaired). The quality of revelation is neither enhanced nor diminished by the loudness of his voice.
