Monotony is when you have too much of a boring thing: one tone of voice going on and on, one piece of flat music playing over and over, one infomercial droning on and on. There is no variety in monotony.
Monotony goes back to the Greek root monotonos, which comes from mono-, "single," and tonos, "tone." One tone only equals monotony. A day with a lot of repetition, or monotony, is humdrum. When you get too much of the same boring, one-note thing, you experience monotony. Monotony. Monotony.