How do I know where the resistor would be placed if I were prototyping?
It is all part of the design knowing if you need a resistor to limit any current on not.
Basically the switch resistor is a pull up or pull down resistor, these are needed.
The arduino can not supply enough current to drive a low impedance speaker, the current must be kept below 40mA. So an 8 ohm speaker would give you a current of 5 / 8 = 625mA if connected up directly. This will damage your arduino. So the maximum load on a pin would be 5 / 0.04 = 125 Ohms, therefore you need a 125 - 8 = 117 ohm resistor in line with an 8 ohm speaker. That is going to make the volume very low, but that is another problem.