What does MATLAB display when plotting a sound signal? When  you load the speech file 'mtlb' in MATLAB (which is by the way the voice of a woman saying the word 'MATLAB'), and plot the data, you get the following plot:


My question is this: what does the y-axis represent? What is MATLAB actually plotting here? Thanks!
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.