An even wilder guess: the speed of sound varies with air density.
https://en.wikipedia.org/wiki/Speed_of_sound#Altitude_variat...
If they were calibrated assuming a certain distance from the microphone that "hears" what the wearer's ear is hearing and the ear itself, then it's possible a change in air density could position the area of highest constructive interference at the eardrum instead of the intended destructive interference for some frequencies.
The pressure difference shouldn’t be significant enough in modern jets? Cabin altitude is around 6000-8000’ - we would hear complaints from a few major cities. Humidity is much lower in aircraft though.
The speed of sound varies with air temperature, which is what the linked graph shows.
Technically the speed of sound does vary with density, but as you change altitude there's also a change in pressure which exactly cancels that out. In the end only temperature and gas composition alter the speed of sound.
As long as you're inside the plane (and hopefully it's not 217 K or -70 °F, per the graph) then the speed of sound should be unchanged.