Loudly is the usual adverb from the adjective loud:loudly 为形容词 loud 的常用副词：
The audience laughed loudly at the joke.观众听到这笑话大笑起来。
Loud is very common as an adverb in informal language. It is nearly always used in phrases such as loud enough, as loud as or with too, very, so, etc:loud 作非正式用语常为副词，几乎总是用于 loud enough、as loud as 等短语中，或与 too、very、so 等词连用：
Don’t play your music too loud.你播放音乐声音别太大。
I shouted as loud as I could.我声嘶力竭地喊着。
Louder is also used in informal styles to mean ‘more loudly’:louder 亦作非正式用语指以更大的声音：
Can you speak louder?你说话大声些行吗？
Out loud is a common adverb meaning ‘so that people can hear’:out loud 为常用副词，意为出声地、大声地：
Can you read the letter out loud?你把信念出来行吗？
He laughed out loud at his own joke.他对自己的笑话大笑起来。
Aloud has the same meaning but is fairly formal. It can also mean ‘in a loud voice’.aloud 意义相同，但相当正式，亦含大声之义。