Some verbs are an exception to the general rule that objects are in the accusative case.
Verbs which involve an action of the five senses or of the mind often take a genitive object (GGBB p131). Note that φωνῆς (genitive, singular) is the object of the verb ἀκούουσιν.
τὰ πρόβατα τὰ ἐμὰ τῆς φωνῆς μου ἀκούουσιν, καγὼ γινώσκω αὐτὰ καὶ ἀκολουθοῦσιν μοι…
Likewise, some verbs take their object in the dative case. (GGBB p171)