A fool's speech is represented by his "mouth." AT: "as he finishes talking, he speaks wicked madness" (See: [[rc://en/ta/man/translate/figs-metonymy]])
The writer asks this question to emphasize that no one knows what will happen in the future after one's death. This question can be written as as statement. AT: "No one knows what is coming after him." or "No one knows what will happen after he dies." (See: [[rc://en/ta/man/translate/figs-rquestion]])