大声(dà shēng) - loudly, out loud; in a loud voice in Mandarin

大声 (dà shēng) describes the volume of speech or a noise. It sits in front of the verb, not after it: 大声说 (speak loudly), 大声喊 (shout), 大声笑 (laugh loudly). Putting it after the verb sounds wrong to a native ear.

The trap for English speakers is treating it as an adjective on its own. You cannot say 他很大声了 the way you would say "he was loud". Attach it to a verb, or use 声音很大 (the sound is very loud) when you want to describe the noise itself rather than the action.

To ask someone to speak up, 大声一点 (a bit louder) is the everyday phrase. Its opposite is 小声 (xiǎo shēng, quietly), built the same way.