"talk softlier" would be the correct english, and even then it sounds antiquated as hell
let me check raws but i highly doubt bro is speaking in old chinese
edit: no, it's just an informal way of saying it (小点声), in which case
"quiet down" or "pipe down" or "not so loud!" would all suffice, but "talk softlier" is just not it.
It's especially bad because apparently ED and TL didn't go over this one to figure out exactly what the issue is