Albert Barnes Commentary Hebrews 12:19

"and the sound of a trumpet, and the voice of words; which [voice] they that heard entreated that no word more should be spoken unto them;" — Hebrews 12:19 (ASV)

And the voice of a trumpet (Exodus 19:19). The sound of the trumpet amid the tempest was suited to increase the terror of the scene.

And the voice of words, spoken by God (Exodus 19:19). It is easy to conceive the awe that must have been produced by a voice uttered from the midst of the tempest, so distinct as to be heard by the hundreds of thousands of Israel, when the speaker was invisible.

Which voice they that heard (Exodus 20:18–19). It was so fearful and overpowering that the people earnestly prayed that if they must be addressed, it might be by the familiar voice of Moses and not by the awe-inspiring voice of the Deity.