The word "avail" (a) is not related to speaking. It means to profit or take advantage of. An example of the word avail used in a sentence is:
He availed himself of the opportunity to learn from his university professor.
The words "discourse", "interjection", and "voluble" are all related to speaking; each with a different meaning. The word discourse relates to a political discussion or written or spoken communication.
The word "interjection" is used to express an emotion or sentiment on the part of the speaker. A good example is "Hey! Come back here." The interjection is the word "Hey". The word "voluble" describes a person who speaks fluently, or talks incessantly.