'read aloud' = 'read outloud', as in a talking voice.
Note that you do not have to be reading TO someone else. Some people read aloud/outloud when reading by themselves. In fact, for *centuries* people read outloud whenever they read, whether they were alone or not. This was because oral language still dominated the culture.
By contrast, 'read loudly' means to read 'in a loud manner'. It could be that your 'reading aloud' is done too loudly. Someone could ask you not to read so loudly. Or someone could ask you to read 'more loudly' because they cannot hear you.