I'm not sure how accurate that is. From what I can tell, only about half of states have some region that requires emissions testing for some vehicles.

https://en.wikipedia.org/wiki/Vehicle_inspection_in_the_Unit...

For example, Washington requires testing in part of 5 counties (out of 39), for non-hybrid, 4-wheeled vehicles older than 2009. But even that is misleading, as the emissions testing program is being discontinued entirely at the end of the year.

So even if they wanted to just add noise testing (it sounds so simple!), most cars don't get annual testing, and in another 6 months, none will. The state won't even have facilities for it.

The bigger issue I see is that emissions is generally seen as bad by everyone. Most people don't spend money to mod their car to pollute more, so if you test cars once a year, it's probably going to be OK the rest of the year. People do mod their cars to make more noise, though, so if you test once a year, they're going to fix the noise for the test, and then put it back the next day.

To fix this problem, you need to be able to catch people on the road.