The Microphone (MCA-BTA) and the Microphone channel on the Go Direct^® Sound Sensor (GDX-SND) 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 (Microphone (MCA-BTA)) to measure loudness in dB, use the Sound Level Meter (SLM-BTA, discontinued), Sound Level Sensor (SLS-BTA), or Sound Level channels on the Go Direct^® Sound Sensor (GDX-SND) instead.