VOVSOFT Noise Detector has a noise threshold level that can be modified via the slider. You can also change the tolerance time (in seconds). It includes three warning sounds to choose from: a beep and a "shhh" spoken by a male or a female. The warning sound is played when the noise level gets too high, exceeding the threshold.
The microphone can be automatically started at launch and minimized to the systray to keep it unobtrusive. VOVSOFT Noise Detector would appear to be geared for classrooms, but it can be used in any situation where you need to keep the noise levels down, like outside home recording studios or even your home office when taking a zoom meeting to let everyone know to "keep it down out there!"