Revelation 10:3

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

RBT Greek Interlinear:

Strongs 2532; kai; καὶ; And; Conj

Strongs 2896; ekraxen; ἔκραξεν; he croaked [as a raven]; V-AIA-3S

Strongs 5456; phōnē; φωνῇ; a voice; N-DFS

Strongs 3173; megalē; μεγάλῃ; mega; Adj-DFS

Strongs 5618; hōsper; ὥσπερ; just as; Adv

Strongs 3023; leōn; λέων; a lion; N-NMS

Strongs 3455; mykatai; μυκᾶται; is mooing; V-PIM/P-3S

Strongs 2532; kai; καὶ; and; Conj

Strongs 3753; hote; ὅτε; when; Adv

Strongs 2896; ekraxen; ἔκραξεν; he croaked [as a raven]; V-AIA-3S

Strongs 2980; elalēsan; ἐλάλησαν; have chattered; V-AIA-3P

Strongs 3588; hai; αἱ; the; Art-NFP

Strongs 2033; hepta; ἑπτὰ; seven; Adj-NFP

Strongs 1027; brontai; βρονταὶ; thunders; N-NFP

Strongs 3588; tas; τὰς; the; Art-AFP

Strongs 1438; heautōn; ἑαυτῶν; themselves; RefPro-GF3P

Strongs 5456; phōnas; φωνάς; voices; N-AFP

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 chirped the voices of themselves.

Caws/Shrieks
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)

chirping of the "akris"...

ἀκρίς - grasshopper, locust, cricket

The Swarming one, Ravaging/Consuming one, the night crickets, There was one word for all the types.

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

ESV Translation:
and called out with a loud voice, like a lion roaring. When he called out, the seven thunders sounded.

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