The TR text has the singular form, he said whereas the MT and NU texts have the plural form, they said. If the former, then the mighty angel was speaking. If the latter, then perhaps both the voice from heaven and the mighty angel or several angels spoke in unison.1
You must prophesy again
You must is δεῖ σε [dei se] which indicates necessity, often to attain a certain intended result.2 John is told to prophesy again. That which he has been relating up to now is prophetic, not some veiled political document in an apocalyptic genre. See The Genre of the book of Revelation. See Audience and Purpose. Victorinus, who wrote the first commentary on Revelation, understood this phrase to indicate Johns subsequent release from Patmos for how could John deliver what he must prophesy if he were to remain on Patmos?
Victorinus [d. c. A.D. 304], who wrote the first commentary on Revelation . . . at Revelation Rev. 10:11+ notes: He says this, because when John said these things he was in the island of Patmos, condemned to labor of the mines by Caesar Domitian. There, therefore, he saw the Apocalypse; and when grown old, he thought that he should at length receive his quittance by suffering, Domitian being killed, all his judgments were discharged. And John being dismissed from the mines, thus subsequently delivered the same Apocalypse which he had received from God.3about many peoples, nations, tongues, and kings
The fourfold designation: peoples, nations, tongues, kings, indicates the global scope of the message John is prophesying. See Four: the Entire World, the Earth. Both Ezekiels scroll and Johns book are closely related. (See commentary on Revelation 10:2.) Both contain prophecy. However, a significant difference occurs between what Ezekiel and John ingest: Ezekiel eats a message intended for Israel, but John eats a message for all nations. Ezekiel is told to prophesy to the house of Israel, not to many people of unfamiliar speech (Eze. Eze. 3:6), whereas John must prophesy again about many peoples, nations, tongues, and kings (Rev. Rev. 10:11+). The message of Revelation is about a multinational, multiethnic population. It is global in nature and cannot be restricted to the events of the A.D. 70 destruction of Jerusalem by Rome as preterists contend. It is no one Empire or Emperor that is concerned in the prophecies of the second half of the Apocalypse; not merely Rome or Nero or Domitian, but a multitude of races, kingdoms, and crowned heads.4 See commentary on Revelation 1:7. The group John is to prophecy about includes those who dwell on the earth, who view the bodies of slain witnesses (Rev. Rev. 11:9+). This is the same group which an angel preaches the everlasting gospel to (Rev. Rev. 14:6+). These are the ones upon which the harlot, Babylon, sits (Rev. Rev. 17:15+). About is ἐπι [epi] which can also be translated against (Luke Luke 12:52-53). For much of what John relates is both about and against the earth dwellers around the globe.
1 They, who told John he must prophesy, we may surmise were heavenly watchers (as in Daniel Dan. 4:13, Dan. 4:17): for the mind of God as to earthly judgments and prophetic programs is well known by those dwelling in the light of heaven (compare Revelation Rev. 7:13+, Rev. 7:14+; Rev. 11:15+, Rev. 21:1+; Rev. 22:9+).William R. Newell, Revelation: Chapter by Chapter (Grand Rapids, MI: Kregel Publications, 1994,c1935), Rev. 10:11.
2 Frederick William Danker and Walter Bauer, A Greek-English Lexicon of the New Testament and Other Early Christian Literature (Chicago, IL: Uersity of Chicago Press, 2000), 172.
3 Mark Hitchcock, The Stake in the HeartThe A.D. 95 Date of Revelation, in Tim LaHaye and Thomas Ice, eds., The End Times Controversy (Eugene, OR: Harvest House Publishers, 2003), 133.