A.T. Robertson Commentary Revelation 10:3

"and he cried with a great voice, as a lion roareth: and when he cried, the seven thunders uttered their voices." — Revelation 10:3 (ASV)

The seven thunders (α επτα βροντα). A recognized group, but not explained here, perhaps John assuming them to be known. For βροντα see already 4:5; 6:1; 8:5. In Ps 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 Joh 12:28 the voice of the Father to Christ was thought by some to be thunder.