Industrial automation monitoring is a major commercial application. I was going to look at https://www.minidsp.com/products/usb-audio-interface/uma-16-... for doing it. Do you know what limits of detection you could get, and on what equipment?
Monitoring structural vibrations is also useful, and I think is an ongoing research area. I mention this because it's possible to sell it and research it at the same time.
What about synchronized cameras in different locations?
The problem I have trying to find the niche application is that there's not much (at least, that I could think of) where you can have high quality audio data, but where a simple camera wouldn't work. Also, full imaging (as opposed to just tracking the largest/loudest source via TDOA) is quite different math, stupendously more computationally intensive.