Hello! Thanks to those who forwarded this to me. At the time this question was posted, I was sleeping, and until now have been occupied with my vocation of Audiology, or getting to/from the place where I practice.
I really don’t have much to add, because most of my points have been made. The ability to detect vibration in the space around us and our own movement are very primitive senses that were crucial to survival. There are many creatures that have little to no use of vision but have highly keen acuity for sound, vibration and movement. As you point out yourself, when we’re sleeping our ears are constantly aware of what is happening in our environment, while our eyes aren’t not doing anything but bouncing around in REM sleep. The ears are they eyes in the back of our head and awareness beyond the 4 walls or whatever else might obstruct our vision. We humans give them far less credit than they deserve.
Historically, there were very few noises that have been loud enough to be damaging until industrialization. The sudden percussive sounds that might occur in the environment are offset by the reflex of the tensor tympani muscle which tightens the eardrum to muffle the sound slightly. For more prolonged loud noises there is the temporary threshold shift, which was recently determined to be a reflexive action and protective mechanism from loud sound. Those two involuntary reflexes were able to handle most of the potentially damaging sounds in the environment before man-made machinery came along.
Beyond that is the voluntary action of covering one’s ears with one’s hands. Simply pushing your finger to block the tragus over the ear canal opening is fairly effective, and something that we do reflexively, and voluntarily.
The system is very effective unless we choose to expose ourselves to louder sounds. As the sources of noise in the environment become more numerous in industrialized cultures, we tend to turn up the things we are listening to, and we’ve become increasingly desensitized to noise. Therefore, we have fooled ourselves that because we can tolerate a sound it is not potentially damaging. Now we have people needing hearing aids in their 50s and 60s because they were too cool or tough to wear hearing protection. The people in their 70s and 80s are the ones who we learned about noise damage from. Occupations with risks for noise-induced hearing loss include dentists, mechanics, seamstresses, housekeepers, hairstylists, and fitness instructors (I’ve worn earplugs to Zumba class) – not just soldiers, rock stars and construction workers.