Signal and Space/Time
My understanding of your question is that you have a small misunderstanding here. In the definition it is not said time and space. A signal can vary in time or space. Some signals vary with time, as your example human voice that varies over time in air pressure (or equivalently voltage), some vary with space, like image and some vary with both time and space such as video. I feel,  your misunderstanding in human voice example is that you believe there should be an space dimension too in the game.