Sound pollution is a matter of public concern which is already regulated by the state.
An appropriate study takes weeks or months, since it would still be bad news to find out after the fact that one week out of the month or every other month has serious noise problems. -- which is an unreasonable amount of delay to stick on the private transaction (in particular, as a buyer you'll be sniped by someone less cautious).
Moreover, the data needed is essentially the same for all properties in an immediate area. This is a natural shared cost, and redoing it once off for each would be extraordinary wasteful.
Finally-- this entire thread sub-thread is about reducing state intervention by buyers being empowered to not purchase places where they're going to have problems with the noise. They can't do that without the information, they can't gather the information themselves on a reasonable time scale. The buyer can't demand the seller provide it (because by the time they meet the seller it's too late to start collecting it).
I don't see this as too much different from things like flood maps (which require rather expensive flyover lidar and historical watershed information, etc. to construct) that cities already provide.
> Lastly, many such maps already exist:
Yes, I'm aware. Which is also why I didn't think it was unreasonable to suggest. (though the DOT maps are understandably focused just on highway noise).