How to make button in Title menu

In my game, I start the gameplay after clicking the left mouse button:

    if event is InputEventMouseButton:
      if event.button_index == MOUSE_BUTTON_LEFT and event.pressed:
        if game_running == false:
          start_game()

But I would like to add a mute button to the main screen. But when you add such a button, when you click on it, the gameplay begins. How can I fix this?
I use tuttorial -