Bhashya
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--