a cloud - not one of our watery clouds, but the Shekinah-cloud (see on Mat 23:39), the pavilion of the manifested presence of God with His people, what Peter calls “the excellent” of “magnificent glory” (2Pe 1:17).
a voice - “such a voice,” says Peter emphatically; “and this voice [he adds] we heard, when we were with Him in the holy mount” (2Pe 1:17, 2Pe 1:18).