speak vs discourse

speak is not a noun while discourse is a noun.

Word	Noun	Adjective	Verb	Adverb
speak	No	No	Yes	No
discourse	Yes	No	Yes	No

As verbs, discourse is a hyponym of speak; that is, discourse is a word with a more specific, narrower meaning than speak:

speak (noun)	discourse (noun)
	an extended communication (often interactive) dealing with some particular topic
	extended verbal expression in speech or writing
	an address of a religious nature (usually delivered during a church service)

speak (verb)	discourse (verb)
express in speech	talk at length and formally about a topic
exchange thoughts; talk with	carry on a conversation
use language	to consider or examine in speech or writing
give a speech to
make a characteristic or natural sound