For the purpose of detecting a defect or failure in a machine, it is often practical to physically listen to any abnormal noises that a machine might be making, and even use a simple stopwatch to determine the frequency of these noises. This practice often gives clues as to what part of a machine is having problems.
For the purposes of detecting failure in a large plant, filled with many machines that might not have many sensors, one can also often isolate a machine with a problem by listening to abnormal ambient noises, and tracing them back.
Is there any sort of practice in which a microphone is placed at a certain location to detect the "ambient sound signature," and then is used as a sort of sensor for problems in plants? I can imagine a baseline being made with this microphone and then used as a reference for abnormal ambient noises, then paired with a monitoring camera to detect whether there is any explanation for non-regular noises.