Android Phone Option
The android phone option with a decibel meter app would end up being very similar to the ESP8266 option that @vseven mentioned, except that the app would already be created. They are very similar sensors, it’s just that some android phones come with them already built in. You wouldn’t use it with a mobile plan, just Wi-Fi. So that’s something that some people have used in the past, which is one reason I linked to the other thread.
As far as the Netatmo weather station, it’s an environmental meter that includes a sound sensor, and you can use IFTTT to trigger when The noise goes above a level that you specified. So it should meet the legal requirements in most US jurisdictions, and you can get integration with smartthings through IFTTT. And there’s no monthly subscription. You may not need the temperature and humidity sensors, but it is the quickest no subscription way to get what you asked for. But the initial cost may be higher than what you wanted to pay.
(The baby monitors are mentioned in the other thread because that one isn’t specific to rentals, just to noise monitors. Also some phone/tablet-based baby monitor apps can be set to only report at a certain noise level, so they might work as well. Obviously, as you mentioned, not the ones that are doing recording or that are used to listen in.)
I’m sorry if it was confusing: the reason I linked to the Airbnb page that listed the three devices you had mentioned was because it was an Airbnb page discussing their policies. So it was intended as an example of the policies, not the specific devices. Again, I’m sorry if I didn’t make that clear. My bad.
As far as what those devices are, as far as I can tell, they’re all just acoustic sensors wrapped in an easy to use package. But they are designed to fit into their own subscription services and I don’t think you’ll be able to use them without those services.
The alternatives are the ones we’ve just discussed. Get your own sensor and, as @vseven suggested, hook it up to something with a radio that can report to smartthings. That could be an arduino, an ESP device, a raspberry pi: any device that can accept sensor input and transmit a message in a form that smartthings can understand. But that one will be quite a bit of set up work.
Or you can look for a device that already includes that kind of sensor and could be used to communicate with smartthings. That would save you a couple of steps in the set up. Again, some android phones can do this, or, if you want to spend more money but have a very easy set up, you could probably have this working in about five minutes with the netatmo environmental monitor device.
It would be great if there was a standalone noise level sensor packaged in a way that you could integrate it with smartthings. IHome used to make one, but It didn’t sell well and it’s no longer available. If you see one, definitely let us know, because I’m sure you’re right that there are many people who would like this these days.