speak louder可以,表示说得更大声些.speak loudly则表示大声说话,2者意思不一样.
以下是权威例句：
1.The unions say they are ready to accept concessions,yet their actions speak louder than words.
WSJ:Scott Walker:Why I'm Fighting in Wisconsin
2.So,yes,I do think that words are important and words matter,but actions speak louder than words.
CNN:The CNN Democratic presidential debate in Texas
3."As summer moves on and people entrench themselves into maps more and more,we will speak louder," Velez said.
CNN:East Los Angeles