What are the units (of pressure) for the microphone?

The microphones are uncalibrated, which means that y axis on microphone waveform graphs has arbitrary units. What you see graphed is the voltage from the microphone. That depends on the sensitivity of the microphone (which varies somewhat with frequency), and the gain of the amplifier (which varies somewhat with frequency).

If we were to calibrate the microphone, the units would be those of pressure, such as kPa.

You cannot use the microphone to measure loudness in dB, use a Sound Level Meter, order code SLM-BTA, $165 instead.