Of the verb jñā , when not used in the sense 'to know', the instrument takes the 6th-Case-affix.,
Ex. मधुनो जानीते 'he engages in sacrifice with honey'. So also सर्पिषो जानीते. The verb ज्ञा when not meaning 'to know', has the significance of 'to act, or to engage in;' or it may denote 'false knowledge'; as सर्पिषि रक्तः प्रतिहतोवा. Compare 1.3.45.
But not so here स्वरेण पुत्रं जानाति 'he recognises the son by the voice.',