What does MATLAB display when plotting a sound signal?
What you are observing is the digital representation of the voltage, which in fact represents the acoustic pressure.
Workflow would be something like:


Vibrating larynx is producing Acoustic Pressure [Pa]
Variations of that pressure are converted by the microphone into Voltage [V]
Voltage is being sampled and quantized by the ADC (Analog to Digital Converter) and represented as series of numbers.


Now knowing your microphone's sensitivity [mV/Pa] you can relate the voltage and acoustic pressure. Additionally if ADC's parameters are known (or measured) then you can directly relate sample values to Pascals.