Actually, I think they are all wrong. Talk to: The speech is in one direction. One, two or more people may talk TO an audience, as long as the audience does not return any conversation. However, if two or more people have an audience and the audience responds with questions, perhaps, we would use 'talk with', because the speech is in two (or more) directions. It is the number of directions in which the speaking flows that determines 'talk to' or 'talk with'.

And also http://www.italki.com/question/208264

Brainer, es la misma diferencia entre "hablar con" y "hablar a" I need to talk with Cristobal about this Necesito hablar con Cristóbal sobre este asunto (Quiere decir que va a havber un intercambio de opiniones, quieres saber lo que Cristóbal dice. En caso de que quieras comprar un coche y Cristóbal te pueda hacer una recomendación) Pero: I need to talk to Cristobal about this. Necesito hablarle a Cristóbal sobre esto. (Quiere decir que vas a decirle algo a Cristóbal, le vas a dar información, sobre lo cual no es necesario que responda para emitir opinión o dar información, puede ser por ejemplo que va a perder el empleo o que ganaste la lotería).

Personally, I think English speakers use the two pretty much interchangeably. The only slight difference in my mind is that if you use "with" there is probably a two-way conversation taking place as in "I talk with my Mom every week." whereas I would probably say "I talked to the audience" if I was trying to say that I gave a speech in which I was doing the talking and the audience was listening. And I totally agree with Frank. The number of people involved in the discussion has nothing to do with what word you choose.

Yes, I agree with Frank, good explanation. I heard people say "talk to" a subject to say they are talking about a subject, but I am quite confident that is slang and quite incorrect.