Vaisesika Sūtra 2.2.27

नित्यवैधर्म्यात् ॥२७॥

nityavaidharmyāt ॥27॥

[नित्य (nitya): Eternal; वैधर्म्य (vaidharmya): inherent nature that is different; आत् (āt): From or due to;]

As it (the sound produced by the word) has an inherent nature that differs from the eternal entity.