The prophets all saying the same thing is spoken of as if they all spoke with the same mouth. AT: "the prophets all declare the same good things to the king" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Here the word "them" refers to "the words of the prophets." AT: "let what you say agree with what they have said" (See: [[rc://en/ta/man/translate/figs-explicit]])