After the verb vad , the ātmanepada is used, when used in the sense of 'speaking articulately in a similar manner'.,
As संप्रवदन्ते ब्राह्मणाः 'the Brahmanas are speaking.' But in संप्रवदन्ति कुक्कुटाः 'the cocks are crowing' it is properly Parasmaipadi.
The sense of the sutra is that when men, who are only capable of articulate speech, speak all in one and the same time, then the verb vad takes the affix of the Atmanepada. When lower animals make a chorus of noise, the verb does not take the Atmanepada
There must be samuchcharana for the application of this rule; when there is no samuchcharana or speaking in a chorus, this rule does not apply, as ब्राह्मणो वदति 'the Brahmana speaks.',