Why does the voice have to be audible?
For those saved by hope, the reason is obvious.
Verily, verily, I say unto you, The hour is coming, and now is, when
the dead shall hear the voice of the Son of God: and they that hear shall live.
John 5:25
the dead shall hear the voice of the Son of God: and they that hear shall live.
John 5:25
24 For we are saved by hope: but hope that is seen is not hope: for what a man seeth, why doth he yet hope for?
25 But if we hope for that we see not, then do we with patience wait for it.
Rom 8:24-25
(See Luke 8:15)