The thunder is described as if it were a person who could speak. AT: "the seven thunders made a loud noise" or "the thunder sounded very loudly seven times"
# seven thunders
Thunder occurring seven times is spoken of as if it were seven different "thunders."
# but I heard a voice from heaven
The word "voice" refers to words spoken by someone other than the angel. AT: "but I heard someone speaking from heaven" (See: [[rc://en/ta/man/translate/figs-synecdoche]])