Thunder in the Scripture is the voice of God. The trumpets is the voice of Jesus. Also both of their voices are spoken by many people or waters.

Now: Re 4:1 After this I looked, and, behold, a door was opened in heaven: and the first voice which I heard was as it were of a trumpet talking with me; which said, Come up hither, and I will show thee things which must be hereafter.

Now I know that everyone thinks that John the disciple wrote this Book of Revelation. But that is just not taught in Scripture. I have written a message already solidly proving with the Word that John the Baptist wrote this Book.

John the Baptist is the man sent from God to bear witness of Jesus and to prepare the way for him.

John the Baptist is the one who bore record of Jesus. So this is the voice of John the Baptist saying these words. John saw that open door. So what was that open door? (Jesus said,  I AM THE DOOR.)

Mt 3:16 And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him:

Mt 3:17 And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased.

Mr 1:9 And it came to pass in those days, that Jesus came from Nazareth of Galilee, and was baptized of John in Jordan. Mr 1:10 And straightway coming up out of the water, he saw the heavens opened, and the Spirit like a dove descending upon him: Mr 1:11 And there came a voice from heaven, saying, Thou art my beloved Son, in whom I am well pleased.

John 1:33 And I knew him not: but he that sent me to baptize with water, the same said unto me, Upon whom thou shalt see the Spirit descending, and remaining on him, the same is he which baptizeth with the Holy Ghost.

1:34 And I saw, and bare record that this is the Son of God.

These verses clearly prove that when John baptized Jesus he saw the heavens opened.