To the voice of the first sign; to the voice or word of God delivered and confirmed by the first sign. For Moses did not make dumb shows before them, but acquainted them with the mind of God therein. Or he saith the voice, to note that God's works have a voice to speak to us, which we must diligently observe. See Micah 6:9.
Related Commentaries of Exodus 4:8
Exodus 4:8
8 And it shall come to pass, if they will not believe thee, neither hearken to the voice of the first sign, that they will believe the voice of the latter sign.