Smartphones can be used for a lot of different functions with many people using older phones as dash cams, dedicated music players, etc. Developer Aiuspaktyn recently published an application called Auto Recorder to XDA Labs that will start and stop recording audio when it reaches and drops below certain levels. This can even let you use your current phone as a voice operated switch, also known as VOX or Voice Operated eXchange. The base Auto Recorder app is free but the pro version comes with additional features.

Check out Auto Recording directly from XDA Labs