Starving bats. Bats keep moth and mosquito populations down in some areas. They use ultrasonic acquisition of their prey.

Rodents less likely to nest near you may be a positive thing.

Great story here:

he said, "Watch the moths." It turns out the moths, through evolution, had developed their own electronic countermeasures to jam the bat radar

https://steveblank.com/2009/03/23/if-i-told-you-i%E2%80%99d-...

A lot of confused hearing aid users.

Currently wearing noise cancelling headphones and wondered what would happen with them, but hearing aids would be a much bigger concern. I would wonder if since hearing aids are so focused on human hearing that they would have filters that would block out any sounds outside the human hearing range.