The seven thunders (hai hepta brontai). A recognized group, but not explained here, perhaps John assuming them to be known. For brontai see note on Rev 4:5; and note on Rev 6:1; and note on Rev 8:5. In Psa 29:1-11 the Lord speaks in the sevenfold voice of the thunderstorm upon the sea.
Their voices (tas heautōn phōnas). Cognate accusative with elalēsan and heautōn (reflexive) means “their own.” In Joh 12:28 the voice of the Father to Christ was thought by some to be thunder.