Is there a naturally occuring sound pressure level reference? There are lots of devices that purport to measure the absolute value of sound pressure levels.  Here is an example, here's another, there's also this iPhone App.  Putting such devices side-by-side in a sound-filled environment usually yields a discrepancy between the decibel readings of each device.  This inevitably leads to the question "Which reading is correct?"
If we were similarly measuring temperature with a collection of thermometers, there are a variety of naturally-occurring physical references against which to verify or calibrate the thermometers.  For example, you could use the triple point of water or the critical point of another substance.  
So, for thermometers, we have a naturally occurring phenomenon with a known temperature to which we can compare our thermometer's reading.
Is there a naturally occurring phenomenon with a known sound pressure level to which we can compare a sound pressure level meter's reading?
 A: I think the best physical references would be at the high end, such as the maximum volume of undistorted sound around 194 dB. There are several other examples of sound pressure level including one on Wikipedia. I don't know enough to know if a thermoacoustic device would meet your requirements, but that's another possibility, anyway.
As an added note, I would point out that you shouldn't even bother with the iPhone App. The calibration is impossible due to manufacturing variability, never mind the frequency response. The gold standard for sound level meters are those made by B&K. I have been told that other analyzers, like the ones you linked, once properly calibrated, can get similar results, though.
A: 
"Which reading is correct?"

That is called measurement uncertainty and that's why we have people called metrologists.

Is there a naturally occurring phenomenon with a known sound pressure level to which we can compare a sound pressure level meter's reading?

Probably not, at least not any practically feasible one at the current level of technology. I base my answer on the fact that sound pressure standards are based on reciprocal calibrations from which they are transferred to sound level calibrators which are electromechanical or electroacoustic devices.
