[xnx]

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.

[clifdweller]

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.

[recursive]

I don't think you can really do that reliably either. Arbitrary consumer device microphones do not have flat frequency response