"And all the people responded together, saying: All that God has spoken we will do! Moses brought their words before God. And God said to Moses: I am coming to you in the midst of a cloud, that the people shall hear Me speak to you... And Moses spoke the words of the people before God." (Exodus 19:8-9)
This is a odd and wonderful thing here, because what are these second words of the people, of which the verse ends "Moses spoke the words of the people before God"? It is not written anywhere in the Torah that they spoke or replied anything else after their first response, "All that God has spoken we will do." What then did Moses speak in their name the second time?
However, know this: Israel complained about Moses instructing them to accept the Torah. They responded: "All that God has spoken we will do?!", meaning, "Shall we be forced to do everything that God tells us? There is no free choice in this! We would be compelled to act in a certain way!" This is the meaning of "we will do," since there is no choice not to do these things, once they are explicitly commanded by God. This is what it means in the verse, "Moses brought their words before God" - he relayed to God the complaints of Israel. Immediately, the Holy One replied: "I am coming to you" - to you, specifically - "that the people shall hear Me speak to you." Again, to you, I will only speak the Ten Commandments to you, saying "I am your God", "You shall have no other gods", and so on [in the second-person singular]. Not directly to the people of Israel, but they only listen to Me speaking to you, and through this they will retain their free will to do as they wish.