What unit is the z axis of a power spectrogram of audio?
The short answer is that without a calibrated data acquisition chain from the original sound as it was picked up from at a microphone and converted to a digital file, it doesn’t have any units.   Its just numbers that fit into 16 bits (or 24 or 8) of dynamic range.  

The number you have is probably proportional to something in nature if the original sound was natural but something coming out of Garage Band is synthetic.