THIS is why you sound monotone

When you move your body, it moves your voice.

So if you don't want to be monotone when you speak, learn to use more hand gestures when you talk. And the simple way to do this?

Play charades when you speak. This forces you to visually describe with your hands what you're saying, which not only makes you more visually dynamic, it makes your voice more dynamic.

Remember, your body is connected to your voice and your voice is connected to your body!