Putting God's Word to rock music or rap should never be used as an evangelistic tool, for the music/rap can actually be a distracter, satisfying the flesh with the rhythm and arrangement. I would say that putting true scripture to music would only be for "believers" as an encouragement and a means of true worship, which we see in the Old Testament Psalms.

But even that music should be and must be honoring to the majesty of the Living God, and not sound like the same music the "world" listens too but only with different words--God's music must be "Holy" (set-apart) to God--unique to Him, for he is Glorious!