How does a sound card translate amplitude values to sound when playing WAV/PCM/FLAC files?
They represent fluctuations from atmospheric pressure.  


Positive values = increases in pressure = speaker cone moving outward
Negative values = decreases in pressure = speaker cone moving inward
Zero = no change in pressure = speaker cone in its relaxed center position = 1 atm


Since we can't hear a constant pressure, the constant pressure is averaged out and ignored, and only the relative vibrations are important.