John Gill Commentary Deuteronomy 4:12

"And Jehovah spake unto you out of the midst of the fire: ye heard the voice of words, but ye saw no form; only [ye heard] a voice." — Deuteronomy 4:12 (ASV)

And the Lord spoke unto you out of the midst of the fire
For the Lord descended on Mount Sinai in a cloud, in fire, and was in the thick darkness, from which he delivered out the ten commands:

you heard the voice of the words ;
distinctly and plainly, not only the sound of them, but the words themselves, and so as to understand what was meant by them;

but saw no similitude ;
not any likeness of the person speaking, by which they could form any idea of him in their minds, which was purposely done to prevent idolatry:

only you heard a voice ;
that was all.