What do you call an out-of-schedule speech in a conference?

Solution 1:

"Extempore/extemporaneous/extemporary," "spontaneous," "unpremeditated," "unplanned," and their synonyms can accomplish this task.

"Extempore," "extemporaneous," and "extemporary" are all defined as:

Spoken or done without preparation. [Lexico]

These three are the most fitting words for this situation. In fact, the example sentence from [Lexico] for "extempore" is: "an extempore speech."

"Spontaneous" is defined as:

Performed or occurring as a result of a sudden inner impulse or inclination and without premeditation or external stimulus. [Lexico]

"Unpremeditated" is defined as:

(of an act, remark, or state) not thought out or planned beforehand. [Lexico]

"Unplanned" is (self-evidently) defined as:

Not planned.

Note: although "impromptu" does apply to this situation, for some it a negative connotation (as Edwin Ashworth pointed out), so I would probably use one of the adjectives I have listed above instead.