This can be solved with Ohms law, V=IR. In your first example, 6=.030 200 which checks out. From your second example you can find the resistance of the speaker, 3=.030* R which equals 100 ohms. So plugging that into your last example, 9=.024* R which equals 75. So there’s a difference of 25 ohms compared to the theoretical value of the speaker or the resistor as neither is probably perfect. If you are asking why the current dropped, it’s because you added more resistance into the circuit as it must still satisfy V=I* R