भगवान् यज्ञपुरुषो जगर्जागेन्द्रसन्निभ: ॥ २३ ॥
brahmaṇaḥ saha sūnubhiḥ
While Brahmā was deliberating with his sons, the Supreme Personality of Godhead, Viṣṇu, roared tumultuously like a great mountain.
It appears that great hills and mountains also have their roaring power because they are also living entities. The volume of the sound vibrated is in proportion to the size of the material body. While Brahmā was guessing about the appearance of the Lord’s incarnation as a boar, the Lord confirmed Brahmā’s contemplation by roaring with His gorgeous voice.