Q: If you give a talk with no audience participation, are you giving a monologue or a discourse?
A: I wouldn't use either word.
"Monologue," according to both The American Heritage Dictionary of the English Language (4th ed.) and Merriam-Webster's Collegiate Dictionary (11th ed.), would imply a dramatic soliloquy, a series of comic stories or jokes delivered by a comedian, a performance by a single actor, or a long speech given by a windbag who's monopolizing a conversation.
And "discourse" doesn't have to mean a talk by one person. It can be a conversation, a long discussion, or simply verbal expression in speech or writing.
How about a "lecture" or perhaps even a "talk"?
The noun “talk,” by the way, comes from talu, the Old English word for “tale.” That, in turn, comes from an even older Old English word, tellan, which gave us the verb “tell.”