Dr Google (DVM) doesn’t have much to say. If we make the assumption, sensible but not validated, that their ears are damaged by the same noise levels as ours, then we should take db readings in our airplanes and take the data to the OSHA standards.
For instance, my Skywagon makes about 100db in the front seat in cruise (it’s quite a bit quieter in the back seat and even quieter in the baggage). According to OSHA, a 100db occupational exposure can be tolerated for 2 hours every work day without damaging hearing.
*shrug*