Mīmāmsa Sūtra 01.01.18

नित्यस्तु स्यात् दर्शनस्य परार्थत्वात् ॥१-१-१८॥

nityastu syāt darśanasya parārthatvāt ॥1-1-18॥

[नित्यः (nityaḥ) - eternal; तु (tu) - but; स्यात् (syāt) - may be; दर्शनस्य (darśanasya) - of darśana (perception/seeing); परार्थत्वात् (parārthatvāt) - because of being for the sake of another;]

However, darśana, i.e. perception of the sound, may be eternal, since it exists for the benefit of another.