Have the meanings of "Low," "Medium," and "High" gain changed throughout firmware updates? For instance, this would cause the same AudioMoth with the same nominal gain setting ("Medium") to record the same sound event at different volumes with different firmwares installed.
No. The gain applied to the microphone signal hasn't changed since the very first firmware version. There have been small changes to the audio processing over the firmware versions most recently with the updated filter routines added in version 1.4.0 but these would have minimal effect on overall volume.