The word "voice" refers to someone who speaks. Alternate translation: "I heard someone saying loudly from heaven" (See: [[rc://en/ta/man/translate/figs-metonymy]])
God saving people by his power is spoken of as if his salvation and power were things that have come. God's ruling and Christ's authority are also spoken of as if they have come. Alternate translation: "Now God has saved his people by his power, God rules as king, and his Christ has all authority" (See: [[rc://en/ta/man/translate/figs-metaphor]])
"have begun to really exist" or "have appeared" or "have become real." God is revealing these things because their time to occur has "come." It is not that they did not exist before.
Fellow believers are spoken of as if they were brothers. Alternate translation: "our fellow believers" (See: [[rc://en/ta/man/translate/figs-metaphor]])