Jamieson, Fausset and Brown on Revelation 10:3

3. as ... lion--Christ, whom the angel represents, is often so symbolized (Re 5:5, "the Lion of the tribe of Juda").

seven thunders--Greek, "the seven thunders." They form part of the Apocalyptic symbolism; and so are marked by the article as well known. Thus thunderings marked the opening of the seventh seal (Re 8:1, 5); so also at the seventh vial (Re 16:17, 18). Wordsworth calls this the prophetic use of the article; "the thunders, of which more hereafter." Their full meaning shall be only known at the grand consummation marked by the seventh seal, the seventh trumpet (Re 11:19), and the seventh vial.

uttered their--Greek, "spake their own voices"; that is, voices peculiarly their own, and not now revealed to men.