Recently I started looking in to this as a backup method of anomaly detection while performing automated testing of our robotics. I concluded that it's actually pretty easy. Depending upon how simplistic your requirements, you can even achieve this cheaply and effectively on a very tiny microprocessor with an attached surface mount MEMS microphone. Additional features like anomalous audio recording, timestamping and alert transmission are not that hard either. No need for a fully-fledged general purpose operating system, or complex algorithms.