Context

A few years ago, I went through a period wherein I Just. Couldn't. Hear. It drove me crazy, and I was half-convinced that everyone was just mumbling. Long story short, I ended up with a diagnosis of stress. That was helpful, because I knew my hearing would improve, but also less than helpful, because... seriously? How could stress force me to constantly ask people to repeat themselves, even when hearing test results showed that I had near-perfect hearing (even super-hearing at some frequencies)?

Yesterday, I finally got my answer. I skimmed an article in The Guardian, (I tried but failed to read it thoroughly before linking to it), and all is explained:

When we feel safe, the muscles in the middle ear contract, with an effect like tightening the skin of a drum. This shuts out deep background sounds, and allows us to tune into the frequencies used in ordinary human speech.

  

But when we feel threatened, it is the deep background noises we need to hear. In evolutionary time, it was these sounds (roars, bellows, the padding of paws or rumble of hooves, thunder, a flood pulse in a river) that presaged danger. So the muscles of the middle ear relax, shutting out conversational frequencies.