Loudly and aloud are adverbs. While loud is generally used as an adjective, it is also used as an adverb in informal writing/speech. Loudly is an adverb of manner formed by adding -ly to loud. Adverbs of manner describe how something happens or how something is done.

“I can’t hear you. Please, speak loudly.” Is correct Loudly is an adjective (speak loudly, eat loudly, walk loudly...) Loud can be used as an adjective (loud music, loud voice) ... sometimes, we use it in place of loudly in informal speech (even though it isn’t 100% correct grammar) ...

Loudly - “The baby is crying loudly.” (Volume - opposite of “quietly”) Loud - “That music is too loud. Please turn it down. (Volume) Aloud - “Please read this aloud.” (Out loud)