kind of a weird one. Sometimes my 2008 Honda Civic EX quietly speaks words after it starts. I've disconnected any audio devices and it still speaks. As far as I can tell, it's speaking French; I can kinda catch a "s'il vous plaît" at the end of whatever it says. I can't find anything online about this, and it's been driving me up the wall.
In comments you've said that the car didn't speak when you fastened your seatbelt before starting the engine, so I'm assuming that this is a "please fasten seatbelt" message.
You also said that you haven't tried this enough times to know if this always works.
So I suggest that you always fasten your seatbelt before starting the engine if you don't want to hear the message. And if it turns out that the car still talks, come back and post a new question.