异口同声

yì kǒu tóng shēng

with one voice

Character by character

异yì
different

口kǒu
mouths

同tóng
same

声shēng
voice

大家异口同声地说“好”。

Everyone said 'Yes!' in unison.

Tone: neutral.

Usually 异口同声地说 or 回答; also for broad agreement.

Learn it in context

This idiom and thousands more are in Learn Chinese with Bao, with audio, pinyin and drills that make them stick.