וַיִּשְׁמַ֧ע יְהוֹשֻׁ֛עַ אֶת־ק֥וֹל הָעָ֖ם בְּרֵעֹ֑ה וַיֹּ֙אמֶר֙ אֶל־מֹשֶׁ֔ה ק֥וֹל מִלְחָמָ֖ה בַּֽמַּחֲנֶה׃
And when Joshua heard the noise of the people as they shouted, he said unto Moses: ‘There is a noise of war in the camp.’
Rashi on Exodus
ברעה means WHEN THEY WERE SHOUTING for they were shouting for joy and making merry.
Or HaChaim on Exodus
וישמע יהושע, "Joshua heard, etc." The expression ברעה needs clarification. What precisely was the "sound" that Joshua heard which he interpreted as the sound of war? In order to understand Moses' response to Joshua one has to have prophetic insight.