I think this is similar to what the Apostle Paul saw and heard when he was caught up to the "Paradise"of the third heaven, but was not allowed to write down the details.
"I know a man in Christ, fourteen years ago—whether in the body, I do not know, or out of the body, I do not know; God knows—such a man, having been caught up to the third heaven. And I know such a man—whether in the body or out of the body, I do not know; God knows—that he was caught up into Paradise, and he HEARD inexpressible words, not being permitted to man to speak." (2 Corinthians 12:2-4)
So, what did the talking thunder say? Maybe we will never know, but here are my thoughts:
I know from experience that I SEE lightening before I HEAR the thunder. And lightning is associated with the return of Jesus to this present earth.
“For just as the lightning comes from the east and flashes even to the west, so will the coming of the Son of Man be.” (Matthew 24:27)
“For just as the lightning comes from the east and flashes even to the west, so will the coming of the Son of Man be.” (Matthew 24:27)
John wrote Chapters 1 through 20 of Revelation to deal with the seven years leading up to the return of Jesus to earth to reign for 1000 years. Then, in Chapters 21 and 22, John writes down what he heard the thunders say to him in Chapter 10. It was the details of a New Heaven and New Earth.
“And I saw a new heaven and a new earth for the former heaven and the former earth pass away…and I saw the holy city, new Jerusalem, descending out of heaven from God...God is with mankind...and He will be brushing away every tear from their eyes. And death will be no more, nor mourning, nor misery; they will be no more, for the former things have passed away" (Revelation 21:1-4)
In Chapters 8 and 9 of Revelation, six angels had SHOWN John details of what will happen during the last few years before the return of Jesus. Then in Chapter 10 he HEARD the "seven peals of thunder" tell him what would happen after the return of Jesus. Yet, John was not allowed to write down what they said until he had finished writing down more of the details of events of the years leading up to Christ's return. These details are written down in Chapter's 11 through 20.
“…in the days of the voice of the seventh angel, when he is about to sound, then the mystery of God is finished as He preached to His servants the prophets.” (Revelation 10:7)
So, I think the thunders told John about the reconciliation and salvation of all humanity later revealed to the Apostle Paul by the risen Jesus. The prophet Isaiah called it a time when humanity will “rejoice and be glad in His salvation”.
“And it will be said in THAT DAY, Behold, this is our God for whom we have waited that He might save us. This is the LORD for whom we have waited; Let us rejoice and be glad in His salvation." (Isaiah 25:9)
“And I saw a new heaven and a new earth for the former heaven and the former earth pass away…and I saw the holy city, new Jerusalem, descending out of heaven from God...God is with mankind...and He will be brushing away every tear from their eyes. And death will be no more, nor mourning, nor misery; they will be no more, for the former things have passed away" (Revelation 21:1-4)
In Chapters 8 and 9 of Revelation, six angels had SHOWN John details of what will happen during the last few years before the return of Jesus. Then in Chapter 10 he HEARD the "seven peals of thunder" tell him what would happen after the return of Jesus. Yet, John was not allowed to write down what they said until he had finished writing down more of the details of events of the years leading up to Christ's return. These details are written down in Chapter's 11 through 20.
“…in the days of the voice of the seventh angel, when he is about to sound, then the mystery of God is finished as He preached to His servants the prophets.” (Revelation 10:7)
So, I think the thunders told John about the reconciliation and salvation of all humanity later revealed to the Apostle Paul by the risen Jesus. The prophet Isaiah called it a time when humanity will “rejoice and be glad in His salvation”.
“And it will be said in THAT DAY, Behold, this is our God for whom we have waited that He might save us. This is the LORD for whom we have waited; Let us rejoice and be glad in His salvation." (Isaiah 25:9)
The Apostle Paul tells us that the “gospel” given to him by the resurrected Jesus was the mystery of Christ - the secret that all humanity, not just the people of Israel, will be saved.
“When you read this, you can perceive my insight into the mystery of Christ, which was not made known to the sons of men in other generations as it has now been revealed to his holy apostles and prophets by the Spirit. This mystery is that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel.” (Ephesians 3:4-6)
Paul refers to this mystery as “my gospel", which had been “kept secret” in the past.
“Now to Him who is able to establish you according to MY GOSPEL and the preaching of Jesus Christ, according to the revelation of the mystery which has been kept secret for long ages past…” (Romans 16:25-27)
“When you read this, you can perceive my insight into the mystery of Christ, which was not made known to the sons of men in other generations as it has now been revealed to his holy apostles and prophets by the Spirit. This mystery is that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel.” (Ephesians 3:4-6)
Paul refers to this mystery as “my gospel", which had been “kept secret” in the past.
“Now to Him who is able to establish you according to MY GOSPEL and the preaching of Jesus Christ, according to the revelation of the mystery which has been kept secret for long ages past…” (Romans 16:25-27)
To me, the words of the "peals of thunder" is the good news of the reconciliation and salvation of ALL and that the never ending home of everyone will be the New Heaven and New Earth.