mówić głośniej in English:

Could you speak up, please.

You'll have to speak up a bit

Talk louder. Could you speak up a bit? I can't hear you / It's getting bad - it's time someone spoke up about it / We need to have the right people around us who will speak up if they see us getting out of balance.

You’ll have to speak up because it’s a bad line.

They can't hear you in the back of the room. Please speak up.

[You have to speak up. The people in the back can't hear you.]

speak up, I can`t hear you

Can you speak up?

Somebody has got to speak up.