When Judah began singing praises, what happened to the armies of Ammon, Moab and Mount Seir?

Answer: They destroyed one another

Ammon and Moab turned on the men of Mount Seir, then destroyed one another. Judah found only corpses; no one had escaped.

“The moment they began their shouts and praises, the LORD set ambushes against the men of Ammon, Moab, and Mount Seir who had come against Judah, and they were defeated.”— 2 Chronicles 20:22 (BSB)