Just curious why the temp sensor has 3 pins and the buzzer only two if both of them change the output voltage based on external factors (temp or sound) ?

The temperature sensor works by taking advantage of the change of resistance in a semiconductor when the temperature changes. So it uses a voltage reference in order to convert the variable resistance into a variable voltage. It's quite possible to build a two-terminal temperature sensor, it's just not how the LM34 works.

The buzzer uses a piezoelectric material which directly converts mechanical pressure into voltage, so it doesn't need a voltage reference. There are three-terminal microphones that work on a different principal, it's just not how the Nerdkits buzzer works.