The voice of the first sign. — Not “the voice of Moses witnessed to by the first sign” (Rosenmüller), but the voice, which the sign itself might be regarded as uttering. (Comp. Psalms 105:27, where Moses and Aaron are said to have proclaimed “the words of God’s signs.”) A miracle speaks to men.
They will believe, i.e., most of them. Accustomed to the tricks of the serpent charmers (see Exodus 7:11 and comment ad loc.), the Israelites might be unmoved by the sight of the first miracle. They were then to be shown the second, which would be much more astonishing to them, having no parallel in their experience. This would persuade the greater number. As some, however, might still doubt, a third sign was provided. God is patient with all reasonable doubt.