someone who makes a formal speech to a group of people
The keynote speaker (=main or most important speaker) was Robert Venturi, the architect.
after-dinner speaker (=someone who makes a speech after a formal meal)
someone who speaks a particular language
the part of a radio, sound system etc where the sound comes out
someone who says something:
Pay attention to the body language of the speaker.
an official who controls discussions in a parliament
the politician who controls discussions in the House of Representatives in the US Congress