अस्तमित आदित्ये याज्ञवल्क्य चन्द्रमस्यस्तमिते शान्तेऽग्नौ किंज्योतिरेवायं पुरुष इति ।वागेवास्य ज्योतिर्भवतीति ।वाचैवायं ज्योतिषास्ते पल्ययते कर्म कुरुते विपल्येति ।तस्माद्वै सम्राडपि यत्र स्वः पाणिर्न विनिर्ज्ञायतेऽथ यत्र वागुच्चरत्युपैव तत्र न्येतीति ।एवमेवैतद्याज्ञवल्क्य ॥बृह. ४,३.५ ॥
In plain words
"When the sun has set, Yājñavalkya, and the moon has set, and the fire has gone out, what light does this person have?" "Speech becomes his light. With speech as his light he sits, moves about, does his work and returns. Therefore, your majesty, even where one cannot make out one's own hand, when a voice is raised one goes toward it." "So it is, Yājñavalkya."
The commentators
Bṛhadāraṇyakopaniṣadbhāṣyam· bhāṣya
शान्तेऽग्नौ वाक् ज्योतिः ; वागिति शब्दः परिगृह्यते ; शब्देन विषयेण श्रोत्रमिन्द्रियं दीप्यते ; श्रोत्रेन्द्रिये संप्रदीप्ते, मनसि विवेक उपजायते ; तेन मनसा बाह्यां चेष्टां प्रतिप-
In plain words
Once the fire dies down, speech becomes the light. And by 'speech' here Śaṅkara simply means sound. Sound reaches the ear as its object, and the ear lights up. Once the ear has caught that light, something wakes up in the mind -- a moment of discernment. And it's the mind, working from that spark, that turns outward into action--
Close to the Sanskrit
When the fire has become quiet, speech is the light. By 'speech' here is meant sound. By sound as its object, the organ of hearing is kindled. When the organ of hearing has become fully kindled, discrimination arises in the mind. By that, by the mind, [he] undertakes outward activity--