All of the New Testament can be traced back to the Old Testament. The OT is our source code. Trumpets are the the call to hear an announcement, or a warning call to battle. (Isa. 18:3; 58:1; Heb. 12:19) The best clue of the "thunders" is from the OT at the base of Mt Sinai in the Exodus wilderness. YHWH had told Moses that He would come down upon Mt Sinai and speak with Moses so that all the people could hear and witness what He said. (Ex. 19:9-11.)
"And it came to pass on the third day in the morning, that there were thunders and lightnings, and a thick cloud upon the mount, and the voice of the trumpet exceeding loud; so that all the people that was in the camp trembled." Ex. 19:16 (KJV)
Young's has verse 16 as:
" And it cometh to pass, on the third day, while it is morning, that there are voices, and lightnings, and a heavy cloud, on the mount, and the sound of a trumpet very strong; and all the people who [are] in the camp do tremble."
Where the KJV has "thunders and lightnings" Young's has "voices, and lightnings".
"And mount Sinai was altogether on a smoke, because the Lord descended upon it in fire: and the smoke thereof ascended as the smoke of a furnace, and the whole mount quaked greatly. 19 And when the voice of the trumpet sounded long, and waxed louder and louder, Moses spake, and God answered him by a voice." Ex. 19:18-19 (KJV)
Which verses Young's has as:
"and mount Sinai [is] wholly a smoke from the presence of Jehovah, who hath come down on it in fire, and its smoke goeth up as smoke of the furnace, and the whole mount trembleth exceedingly; 19 and the sound of the trumpet is going on, and very strong; Moses speaketh, and God doth answer him with a voice."
The thunder in Exodus were the utterances and voices from within the presence of YHWH, announcing His coming into the presence of the people. Much as a bailiff announces the entrance of a judge into the courtroom, the thundering voices announced the appearance of YWHW upon Mt. Sinai.
In Rev. 10:3 the strong messenger of verse 1 (Christ) sounded with a loud roar...
" and he cried with a great voice, as a lion doth roar, and when he cried, speak out did the seven thunders their voices;" (YLT)
In Rev 10:1 Christ came down from heaven, from the throne scene arrayed in a cloud (see Matt. 24:30; Mark 13:26), and then in verse 3 the seven thunders speak out. The courtroom "bailiffs" of the throne scene were announcing the judge (Christ) and His coming. Christ had been given the keys to the gates of Hades and of death (Rev. 1:18), and the gates of Hades would not prevail against His church (Matt. 16:18). Christ was the judge coming in the clouds in glory.
So, the seven thunders were announcing His entry into, His pronouncements regarding the judgment battle in Rev. 10:3.
We are not told what those voices said, as John was told to seal them up. So, they were not for our knowledge as any prophesy that is sealed is not to be revealed.
It is most probable that the seven thunders are seven voices of YHWH's "bailiffs," His most prominent courtroom angels who keep order in the presence of His court.