Was this the long Teki'ah mentioned in verse 5?
Malbim: It was not, for if so, they would need to blow Teru'ah immediately afterwards. Rather, they blew like every day. Yehoshua wanted to hint to the Kohanim to blow a long Teki'ah, which is a sign of victory. Beforehand, he needed to inform them how to conduct with the city and its spoils. After the wall falls, the residents will be opposite [Bnei Yisrael], and there will not be time to command and warn.