So is it "who you hear" or "what you hear"?
τίς is an interrogative pronoun, and as a pronoun, it declines according to (1) case, (2) gender, and (3) number. The lexical form τίς is declined in the nominative case, masculine/feminine gender, and singular number.
The Greek text of Mark 4:24 states,
Καὶ ἔλεγεν αὐτοῖς Βλέπετε τί ἀκούετε ἐν ᾧ μέτρῳ μετρεῖτε μετρηθήσεται ὑμῖν καὶ προστεθήσεται ὑμῖν τοῖς ἀκούουσιν
In Mark 4:24, we don't encounter τίς but τί, which is declined in the accusative case (according to the syntax, although the same form occurs in the nominative case) and singular number, but neuter gender instead of masculine/feminine gender.
In English, we use the pronoun "who" when referring to male or female people (persons), but "what" when referring to impersonal objects. Likewise, in Greek, we use the pronoun τίς when referring to people, but τί when referring to impersonal objects.
Therefore, the proper translation of «τί ἀκούετε» would be "what you hear." Had the Greek text been written «τίνα ἀκούετε», then we could translate that into English as "whom you hear."