Hashem instructs Moshe to caution the Jewish People regarding their responsibility to be faithful to the One who spoke to them. The Shema is the basic credo of the Jew, his first declaration of G-d's Unity and the last words to leave his mouth when he passes from this world. When the Jewish People stood at Sinai to receive the Torah, they underwent an experience which was literally out of this world. There was a dislocation of the natural perception of the senses. Sight and sound are very different.
Sight operates instantaneously.
We see through the medium of light. Light is the fastest thing in the universe. It travels at 186,000 miles per second. Sound is relatively slow, moving at about 800 miles an hour. The difference between the speeds of light and sound symbolizes a fundamental difference between the two senses.