Ganaka Vaideha said: 'When the sun has set, O Yagnavalkya, and the moon has set, and the fire is gone out, what is then the light of man?' Yagnavalkya replied: 'Sound indeed is his light; for, having sound alone for his light, man sits, moves about, does his work, and returns. Therefore, O King, when one cannot see even one's own hand, yet when a sound is raised, one goes towards it.' Ganaka Vaideha said: 'So indeed it is, O Yagnavalkya.'

In plain words: The king asks what a person's light is when the sun, moon and fire are all gone. Yajnavalkya says sound is then the light: even when you cannot see your own hand in the dark, you can walk toward a voice.