And when he had cried out, the seven peals of thunder uttered their voices. This occurs in answer to the appearance and cry of the angel. As lightening appears and flashes across the sky, and thunder follows, so now the seven peals of thunder are heard following the appearance of the mighty angel. The thunder answers the lion-like voice of the angel. Note that the thunder and the voice of the angel are not the same.
Thunder is a symbol of judgment but also of revelation .
It reminds us God has revealed Himself in history to man, first in creation and then in various ways through special revelation, i.e., through the holy Scriptures and through the Son. The idea is that thunderstorms are a reminder to man that he should ascribe glory and strength to God and worship God as the Creator King of this world. In Revelation 10, the thunder is heard in a most electrifying message that John was both able to hear and understand. The message was so electrifying and astounding that John, as was his custom with these visions, was about to write down what he heard, but a voice out of heaven, perhaps the Lord Himself, forbids this action. Evidently the message was so awesome that man could not handle it.