Loud Voice, Big Voice
noun, の adjective
A big voice is a loud voice! 大声 usually just describes talking in a big voice, but it can also refer to shouting depending on the situation.
おおごえ
The reading for 声 is ごえ (rendaku'd version of the reading you know: こえ). The reading for 大 is the kun'yomi reading, like with the word 大きい (おおきい). Just imagine a loud voice saying "おお！."
つい大声が出てしまった。
I raised my voice despite myself.
コネ入社だなんて、大声では言えないよ。
I can't say too loudly that I joined the company through a connection.
山の方に大声で「やっほー」と言うと「やっほー」と山びこがかえってきた。
When I said "Yahoo" to the mountains, a "Yahoo" echoed back.