You can speak aloud and not keep your thoughts in your mind because you are allowed to do that. These two words are considered homophones because they sound the same, are spelled differently, and are different in meaning.
Allowed is an adjective or verb, and it means that you are permitted to do something. Aloud is an adverb, which means that you can use your full voice. A sample would be Jake is allowed to drive to the market; however, he is not able to say what he is thinking aloud.
The words allowed and aloud are pronounced the same, which is their only similarity. Aloud literally means out loud. Loud could mean something is up at a higher capacity than which is tolerable. It means sound, so than anything that makes a sound would be loud.
Aloud is a combination of all these different parts in various ways. It is often an adverb, though it can be used as an adjective. Allowed is the former tense of the verb allows. The word allow means to give something.
However, it is relatively more passive then give, since it has the suggestion that someone can have what they are trying to take, rather than it being actively given to them. Allowed is most often used as a verb. However, some verbs may be used as adjectives, which is how allowed is commonly used.