want the other person to speak up? "Could you speak (a little) louder(, please)?" is good, too, although perhaps slightly less idiomatic. Theoretically, you could say, "Could you speak (a little) more loudly(, please)?" but to some people, that sounds horribly pretentious.

Could you speak more loudly. Loudly is an adverb, it modifies the verb speak. Louder is an adjective, since adjectives modify nouns, it would only be correct to use it in a sentence structured as this: You are louder. Or this: The louder dog barks at the mail man more often than the quieter dog. You can use an adjective after a verb that states a sense of being such as: am, are, is, was, were, since that adjective refers to the subject noun: I am louder, But not after a verb of action: I speak loudly.

Could you speak louder? <=

Try: Hey, mate, speak up! Can't hear ya'. Always works for me. :)