Revelation 10:3

καὶ ἔκραξε φωνῇ μεγάλῃ ὥσπερ λέων μυκᾶται· καὶ ὅτε ἔκραξεν, ἐλάλησαν αἱ ἑπτὰ βρονταὶ τὰς ἑαυτῶν φωνάς.

RBT Translation:

μυκᾶται - prop. of cattle, low, bellow

and he cawed in a mega voice, indeed even as a lion is mooing,¹⁵ and when he cawed, the Seven Thundering Ones babbled the Voices of Themselves.

Caws/Shrieks/Babbles
"Indeed even as a lion is mooing..."

"And Pharaoh stood up in the Night, himself and all the slaves of himself, and all of Dual-Siege, and she is becoming, a great shriek within Dual-Siege because there is no house of whom there is no dead one."

(Exodus 12:30 RBT)

λαλέω, to talk, chat, prattle, babble
ἕπου ... καὶ μὴ λάλει – “Come along and stop babbling!” (Aristophanes, Ecclesiazusae 1058)

Julia Smith Literal 1876 Translation:
And he cried with a great voice, as a lion roars: and when he cried, the seven thunders spake their voices.

LITV Translation:
and cried with a great voice, as a lion roars. And when he cried, the seven thunders spoke their sounds.

Footnotes

15

This onomatopoetic word (a word which phonetically imitates or resembles the sound that it describes) was never translated honestly in any translation that we have found. See various parallels on the verse for yourself. The translation of "roar" is simply false. 

The "reputed" Thayer's Greek Lexicon even admits the real meaning, and yet still cheats it:

μυκάομαι, μυκωμαι; (from μύ or μύ, the sound which a cow utters (Latinmugio)), to low, bellow, properly, of horned cattle (Homer, Aeschylus, Euripides, Plato, others); to roar, of a lion, Revelation 10:3.

The Abbott-Smith Greek Lexicon of the New Testament falsely represents it as well:

prop., of oxen (onomatop.), to low, bellow; of a lion, to roar: Re 10:3