What did the seven priests blow as Israel marched around Jericho?

Answer: Rams' horns

Seven priests carried seven rams' horns in front of the ark and blew them as Israel marched around the city.

“Have seven priests carry seven rams’ horns in front of the ark. Then on the seventh day, march around the city seven times, while the priests blow the horns.”— Joshua 6:4 (BSB)