The seven thunders (α επτα βροντα). A recognized group, but not explained here, perhaps John assuming them to be known. For βροντα see already Revelation 4:5; Revelation 6:1; Revelation 8:5. In Revelation 10:29 the Lord speaks in the sevenfold voice of the thunderstorm upon the sea.

Their voices (τας εαυτων φωνας). Cognate accusative with ελαλησαν and εαυτων (reflexive) means "their own." In John 12:28 the voice of the Father to Christ was thought by some to be thunder.