Looking for a way to monitor raw microphone digital output?
So my question is is there a way that the digital output of the microphone can be directly monitored and recorded in units of Volts. Hence, I would use these values to calculate Pascals. Can anyone point me to a (preferably free) program that I can use to do this direct monitoring?


So, in fact, your sound card is a Analog-to-Digital Converter, which does exactly that: convert the voltage coming from a microphone into a stream of numbers. Congratulations, you already have your data acquisition output :)

That stream of numbers has no physical unit (it is "best-case proportional to input Voltage", but it is a number, not a physical entity anymore)– you will need to calibrate it with a couple of known signals. 

To make matters more complicated, the amplitude response of a soundcard can't, technically, be flat. Due to the [Shannon-Nyquist theorem], the microphone voltage signal must first go through a low-pass filter that limits the maximum frequency contained in the signal (you're a physicist: apply Fourier transform to time signal $s(t)$, giving you $S(f)$, find $f_{max}$ for which $S(f&gt;f_{max}) \approx 0$).

Since perfect analog filters don't exist, the suppression "edge" can't be infinitely steep, so you'll see changing effect of voltage on digital numbers with changing frequency.

If you work with only one (or few) frequency(ies), and you have a multiple reference pressure wave source for this(those) at different powers, then go ahead and use the sound card, calibrate using the known powers, and calculate correction factors.

If you need something that actually promises linearity, sound cards with their analog filters optimized for audio perception might not be the tool of choice. Verify your approach with an oscilloscope, then use a suitable data acquisition unit – some digital oscilloscope can record signals for you, there's even digital oscilloscopes that have no display and are only meant for usage with a PC, other people I know use Software Defined Radio (SDR) devices to sample audio/sonar frequency signals instead of RF, yet others have just built their own microcontroller-based data acquisition devices based on 10€ evaluation boards.