1. A voice conversion method of a display apparatus, the voice conversion method comprising: in response to receipt of a first video frame, detecting, by the display apparatus, one or more entities from the first video frame; in response to a selection of one of the detected entities, storing, by the display apparatus, a selected entity; in response to a selection of one of a plurality of previously stored voice samples, storing, by the display apparatus, a selected voice sample in association with the selected entity in a storage unit; and in response to receipt of a second video frame including the selected entity, changing, by the display apparatus, a voice of the selected entity based on the selected voice sample and outputting the changed voice, wherein the detected entities comprise faces of characters included in the first video frame.