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.