There's an important side-topic to this. Bear with me for a moment:
If we look at how the mind processes written language, we see that there are generally 3 parts that memory draws upon. There is 1) the visual appearance of written characters, 2) the auditory pattern that fits with those images, and 3) the meaning of the words.
Basically:
1) Sight
2) Sound
3) Meaning
If you practise enough, you can recall the meanings of things without silently repeating the words in your head (subvocalization). The supression of subvocalization is a technique used by some to help speed-read.
It is possible to have a meaning directly conveyed without sight or sound. So even if someone hasn't directly heard the voice of God, it doesn't mean that there has been an absence of direct communication. Someone can have a meaning 'downloaded' directly into their mind without the sensation of sound or "hearing".