Speak thou, &c. From Deuteronomy 5:28 with Deuteronomy 18:7 we learn that on that very day God promised to send "Him that speaketh from heaven" (Exodus 18:15-18).
Related Commentaries of Exodus 20:19
Exodus 20:19
19 And they said unto Moses, Speak thou with us, and we will hear: but let not God speak with us, lest we die.