If you want to lower the sound level a lot we have to think about the fact that the ear's sensitivity is logarithmic. So, for example, if you want to drop the volume a lot, perhaps a 15 dB reduction is appropriate. Assuming the internal speaker is rated at 3 ohms, and assuming you put a 3 ohm resistor in parallel as recommended, then an appropriate series resistor would be 6.8 ohms. This would drop the voltage to the speaker by a factor of .18, which is about -15 dB. If you don't bother with the 3 ohm resistor in parallel with the speaker, than the series resistor should be increased to 15 or 18 ohms to get the same reduction in volume of 15 dB.