We say someone reads aloud when they speak the words as they read （朗读）. If someone is loud, or speaking loudly, then the sound is too noisy and bothersome. "Please be quiet. You are playing too loudly".

Aloud means you speak something rather than read it or think it. I read the Shakespeare play aloud. Loud means I am speaking where people would probably like for me to lower my voice. Or I am speaking to a crowd. The man was so loud it drowned out my own thoughts. I wished he would be quiet. The crowd was mumbling so the speaker had to speak in a loud voice to be heard. (I hope this helps)