What did Jehoshaphat send out ahead of his army?

Answer: Singers praising the LORD

Singers went before the army praising the LORD, and as they sang, the LORD set ambushes against Ammon, Moab and Mount Seir.

“Then Jehoshaphat consulted with the people and appointed those who would sing to the LORD and praise the splendor of His holiness. As they went out before the army, they were singing: “Give thanks to the LORD, for His loving devotion endures forever.””— 2 Chronicles 20:21 (BSB)