The seven thunders here in Revelation refer to the voice of God as written in Psalm 29.
(Psa 29:3-9 KJV) The voice of the LORD is upon the waters: the God of glory thundereth: the LORD is upon many waters. {4} The voice of the LORD is powerful; the voice of the LORD is full of majesty. {5} The voice of the LORD breaketh the cedars; yea, the LORD breaketh the cedars of Lebanon. {6} He maketh them also to skip like a calf; Lebanon and Sirion like a young unicorn. {7} The voice of the LORD divideth the flames of fire. {8} The voice of the LORD shaketh the wilderness; the LORD shaketh the wilderness of Kadesh. {9} The voice of the LORD maketh the hinds to calve, and discovereth the forests: and in his temple doth every one speak of his glory.