Why can't the guilty in Les's analogy say anything?
The guilty in the analogy cannot say anything because they were guilty, they were sentenced, and they are going to what they deserve. Their guilt and sentence leave them with nothing to say, and their destination is exactly what they deserve.
Les explains that in the analogy the other seven can never say one word because they were guilty, they were sentenced, and they are going to what they deserve. This illustrates God's sovereignty in grace: we were hell-bound sinners just as deserving of punishment, but we escaped because we accepted God's pardon. Les emphasizes that it is only by the grace of God that we are saved, and we can never boast as though we were smart enough or had the wherewithal. The guilty, by contrast, have no basis to speak because their judgment is just.