The ṣ is substituted for the s of svan after vi and ava when the sense is 'to smack while eating'.,
Thus विश्वणति, ष्यष्वणत्, विषष्वाण, अवष्वणति, अवाष्वणत्, अवषष्वाण ॥ That is, he makes sound while eating, he eats with a smack.
Why do we say \when making a smacking sound in eating\? Observe विस्वनति मृदङ्गः \the drum sounds\.,