Or HaChaim on Exodus 19:19/אור החיים על שמות י״ט:י״ט

1 משה ידבר והאלוקים יעננו בקול, Moses would speak and G'd answered him by voice. Moses may have uttered words of song and praise before G'd, much as someone who is received in an audience by a king praises the ruler who has granted him the audience. The קול with which G'd answered may have been the sound of the shofar mentioned in the next verse. This sound was an indication that G'd approved of what Moses told him. The word בקול means "in a loud voice," so that all the people could hear G'd speak with Moses and accept him as a true prophet. I believe that the Torah mentions this to tell us that Moses experienced a promotion when G'd answered him thus although G'd had also answered Moses previously. Our sages in the Mechilta understand the answer the Torah speaks of as the Ten Commandments which G'd was about to address to the people. They say that G'd increased the power of Moses' voice so that he could be heard by the whole nation. I believe that this is a homiletical approach to our verse.