Converting arbitrary mic input number to dB SPL
To calculate the real sound pressure level (SPL) from the sample values from the sound card, you would need several things:


The sensitivity of the microphone (voltage produced for a specific, real sound pressure level.) This is given in the specifications of the microphone.  The rating in the datasheet you linked to is kind of odd.  It says 0dB is 1volt per pascal, then says that the sensitivity is -42dB.  So, that's 0.008V per pascal. Note that it is +-3dB.  So, there is considerable variation in the sensitivity of individual microphones.
You will need the gain of all amplifiers between the microphone and the ADC.  It is unlikely that you will have information on all the amplifiers in the sound card.
You will need to know what voltage the full scale reading of the ADC equates to.  It is unlikely that you will have this information.


Since you can't get much of the information you need, and what you can get isn't precise enough, you will have to go another route.

The usual way would be to compare your microphone/sound card setup to a known sound pressure level.  In other words, you need to calibrate your measurements.


Turn off any automatic gain control in the line in of your sound card (or microphone input.)
Set the gain of the line in (or microphone in) to an easily reproduced setting (full up is easiest.)
Setup a sound source, your microphone, and a calibrated SPL meter so that the SPL pickup and your microphone are at the same distance from the sound source.
Make several measurements (at different volume levels) using both your microphone and the SPL meter.  Note them down.


Using the table of measured values, you can calculate the true SPL from any measurement you make with your microphone - but, you must always use the same gain settings and setup as you used in the calibration setup.