Here "their mouth" is a metonym for their speech, and "heavens" is a metonym for God, who lives in the heavens. Alternate translation: "They speak against God, who is in the heavens" (See: [[rc://en/ta/man/translate/figs-metonymy]])
The word "tongues" represents the people themselves. Possible meanings are 1) "they go through the earth saying bad things about God" or "they go everywhere and boast about themselves." (See: [[rc://en/ta/man/translate/figs-synecdoche]])