from the writings of Rebbe Nachman of Breslov
You can shout and scream with a "still, small voice" without anybody hearing you. Everybody can do this. Imagine the sound of such a scream in your mind. Imagine the shout exactly as it would sound. Concentrate on hearing the scream in your mind until you are literally screaming with this soundless "still, small voice" - and nobody else will hear anything.
This is actually a scream and not mere imagination. Just as there are channels that bring the sound from your lungs to your lips, so there are nerves that bring the sound to the brain. You can draw the sound through these nerves, literally bringing it into your head. When you do this, you are actually shouting inside your brain.
When you imagine this scream in your mind, the sound actually rings inside your brain. You can stand in a crowd of people screaming this way and no- one hears you.
It could be that when you do this, a faint sound may escape your mouth. This is because the sound traveling through the nerves to the brain can also activate the vocal organs, and they may then produce some sound. But it will be very faint.
It is much easier to shout in this way without words. When you wish to express yourself in words, it is harder to hold the voice in the mind without letting a sound escape your lips. Without words, it is much easier.
Rebbe Nachman of Breslov: Sichot Haran #16