Exodus 19:16 And it came to pass on the third day in the morning, that there were thunders and lightnings, and a thick cloud upon the mount, and the voice of the trumpet exceeding loud; so that all the people that was in the camp trembled. - Free Bible Online

      16. on the third day in the morning, that there were thunders and lightnings, &c.--The descent of God was signalized by every object imagination can conceive connected with the ideas of grandeur and of awe. But all was in keeping with the character of the law about to be proclaimed. As the mountain burned with fire, God was exhibited as a consuming fire to the transgressors of His law. The thunder and lightning, more awful amid the deep stillness of the region and reverberating with terrific peals among the mountains, would rouse the universal attention; a thick cloud was an apt emblem of the dark and shadowy dispensation (compare Mt 17:5).
      the voice of a trumpet--This gave the scene the character of a miraculous transaction, in which other elements than those of nature were at work, and some other than material trumpet was blown by other means than human breath.

JFB.