Hello! The difference between these two is subtle. "Loud" is used to indicate the actual noise level of an action. "Aloud" is used to distinguish between "audible" and "inaudible". "Read it aloud" means that you read it to the whole class or to another person, instead of just silently reading it. You basically use your mouth instead of just your brain. So you can not interchange them because the meaning of the sentence would change.