Oh, hey the question was "who should be doing the talking?"
Everyone! I play front and I yell my lungs out. I ask questions, I repeat stuff, I make fun of opponents, I ask when to bump, whatever. Don't sneak around in speedball... it's nekulturny.
A good team (as far as communication goes at least) should all be talking nearly all the time, its just that the center back is the hub of all the communication.