You can ask them politely.

But some people are in the habit of talking loudly (my brother for instance).

And there is a cultural issue with some people as that is what they grew up with. For example, in parts of Europe people in public places tend to talk softly compared to the USA where they are in general louder. While I believe in parts of China they talk rather loudly in restaurants.

So I tend to think this is one of those things up with which you put...