Verse 3.18.6

श्रोत्रमेव ब्रह्मणश्चतुर्थः पादः स दिग्भिर्ज्योतिषा भाति च तपति च भाति च तपति च कीर्त्या यशसा ब्रह्मवर्चसेन य एवं वेद य एवं वेद ॥ ३.१८.६ ॥
॥ इति अष्टादशः खण्डः ॥

śrotrameva brahmaṇaścaturthaḥ pādaḥ sa digbhirjyotiṣā bhāti ca tapati ca bhāti ca tapati ca kīrtyā yaśasā brahmavarcasena ya evaṃ veda ya evaṃ veda || 3.18.6 ||
|| iti aṣṭādaśaḥ khaṇḍaḥ ||

6. The organ of hearing is one of the four feet of Brahman [as the mind]. It shines by the light of the quarters and also radiates heat. He who knows this shines and radiates warmth with his good work, with his fame, and with the radiance he acquires from leading a scholarly and disciplined life.

Word-for-word explanation:

Śrotram eva brahmaṇaḥ caturthaḥ pādaḥ, the organ of hearing is the fourth foot of [manomaya] Brahman; digbhiḥ jyotiṣā, by the light it gets from [its presiding deity,] the quarters; sāḥ bhāti ca tapati ca, it shines and gives heat; yaḥ evam veda yaḥ evam veda, he who knows thus, he who knows thus; bhāti ca tapati ca, shines and radiates warmth; kīrtyā, through good work; yaśasā, through fame; brahmavarcasena, through the radiance that comes from a scholarly and disciplined life. Iti aṣṭādaśaḥ khaṇḍaḥ, here ends the eighteenth section.

Commentary:

The organ of hearing derives its power from the quarters, and through this organ we get knowledge. The results of that knowledge may be direct or indirect. True knowledge, however, is knowledge of Brahman. To emphasize this, ‘he who knows’ is repeated. This emphasis suggests that it is the knowledge of Brahman.

The organ of hearing is our ‘foot’ because it is the means by which we can know Brahman. Those who know the importance of the organ of hearing and use it in the right way, acquire name, fame, and become radiant like Brahman. They will also eventually realize Brahman.