After the verb kṛ preceded by vi , even when the fruit of the action does not accrue to the agent, and when the sense is that of 'making sound' (literally having 'sound' for its object), the ātmanepada affix is used.,
The word karma in the sutra indicates objective case or karaka; and does not express 'action,' as in some previous sutras such as 1.3.24 ante.
As क्रोष्टा विकुरुते स्वरान् 'the birds are making noise,' ध्वांक्षो विकुरुते स्वरान् ॥
The word वि governs the succeeding sutra.
Why do we say \when governing a word expressive of sound in the objective case\? Witness चित्तं विकरोति कामः 'love affects the mind.',