Midrasch zu Schemot 28:9

R. Yosé the son of Hanina stated further that just as the manna, which was actually of one kind, could change into different kinds (tastes), because of the requirements of each of them, so could the voice that went forth change for each of them according to the individual’s ability to hear it, so that no harm might befall him. Whence do we know that the voice divided itself into many sounds? It is said: And all the people perceived the thunderings. The plural thunderings is written, and not the singular “thundering.” Therefore, God thundereth marvelously with His voice (Job 37:5).