I've searched but failed to find anything that works as a standard fiducial-type sound. e.g. With a recording of the sound produced by the fiducial from a set distance (and knowing temperature, pressure, humidity), you could calibrate/adjust recordings.
That is the crux you always need a calibrated sound to calibrate the mic. Best I think this app could hope to do is establish a relative reading. perhaps it could play a set of repeatable tones on the device(assuming being executed from a device that does not have external speakers that can be adjusted/ validate that sound was played from phone speaker and not headphone jack to a dac?) and record those then could replay sounds later to a calibrated device to establish reference. or start recording with some other regular repeatable sound like a vacuum cleaner with video showing how it was positioned for recording.
Please consult with a lawyer, these kinds of "baby's first foray into legal matters" projects can go wrong in silly ways. That is, if you're serious about this being for the purposes of civil suits, etc.