When the Mighty Angel cried with a loud voice, John heard seven thunders utter their voices.

         These Thunders spoke, because John heard what they said, and as he had been commanded to write what he saw and heard, he proceeded to write what the voices of the seven thunders said, but he was told to seal up those things which the seven thunders said and not write them.

                 What they said hasn't ever been revealed, but will be when the time comes to make the revelation.