Verbs which involve an action of the five senses or of the mind often take a genitive DO (GGBB p. 131).
τὰ πρόβατα τὰ ἐμὰ τῆς φωνῆς μου ἀκούουσιν, καγὼ γινώσκω αὐτὰ καὶ ἀκολουθοῦσιν μοι…
Note that φωνῆς (genitive, singular) is the object of the verb ἀκούουσιν.
Likewise, some verbs take their DO in the dative case (GGBB p. 171).
