Vaisesika Sūtra 2.2.28

अनित्यश्चायं कारणतः ॥२८॥

anityaścāyaṁ kāraṇataḥ ॥28॥

[अनित्य (anitya): Impermanent or transient; च (ca): And; अयम् (ayam): This; कारणतः (kāraṇataḥ): Due to the cause or because of the reason;]

And, it (the sound produced by the word) is non-eternal as it has a cause.