The difference between speech and talk is that speech is limited usually to a formal address or the style of speaking (Oxford 2016). Whereas, according to Oxford (2016) talk usually refers to informal speaking between people.
Activity Two: “I’m giving a speech” refers to making a formal statement to an audience. This is usually a very formal form of communication.
“I’m giving a talk” can refer to either the informal communication between people or a formal negotiation or communication between certain parties.
I believe that speech is more formal than “talk” because it is often used when wanting to make a formal statement with the public.
Oxford 2016, Speech, viewed 29 February 2016, http://www.oxforddictionaries.com/definition/english/speech
Oxford 2016, Talk, viewed 29 February 2016, http://www.oxforddictionaries.com/definition/english/talk