I'll say something for other cyclists (professional courtesy). For pedestrians it will depend on my mood. I find that most of the time they won't hear me over their headphones, or they act like deer: some whip their head around and freeze, some get to the right, and some get to the left.
Several months ago i was trying to pass a kid on his left and called out appropriately. He looked over his right shoulder, didn't see anything and kept rotating his head around, causing him to drift left, nearly forcing me into gravel and grass and whatever else was there. He was also carrying his helmet.
Since then, it just depends on my mood and circumstance. If I'm going to pass quickly and the person is far enough over, I usually don't say a thing.