3 Nephi 15:23

and they understood me not that I said they shall hear my voice; and they understood me not that the Gentiles should not at any time hear my voice; that I should not manifest myself unto them, save it were by the Holy Ghost.

They didn’t understand when I said those sheep will hear my voice. They didn’t understand that the Gentiles would never hear my voice. They didn’t understand that I would not show myself to the Gentiles except through the Holy Spirit.

They didn’t understand when I said ‘they’ll hear my voice.’ They didn’t grasp that the Gentiles would never hear my voice directly—that I would only reveal myself to them through the Holy Spirit.