Do you know the phrase “speak up”?

It means “speak louder!”

E.g. 1:

A: Speak up! I can’t hear you!

B: Sorry, but  I can’t speak up! I have a cough and a sore throat!

E.g. 2:

A: Mr Biggs is always telling James to speak up. No one can hear him in class.

B: I know. I think he is really shy.