Is exhaust volume something which varies from place to place? If a cop pulls me over does he have to prove the bike is over a certain decibel level with a meter?

a cop might give you some crap about exhaust but from my personal experience there are alot louder harleys, so they only give you rap if they dont like sport bikes

trust me, they can write you up for loud pipe for excessive noise. Thats how they get around not having specific decibles. I have several bikes and have seen first hand tickets given to Harley bike with loud pipes.

they give you "rap" if you are being a douche about it, more than likely. In some states, in addition to having a decibel meter reading, the officer can make an aural estimation. If your pipe is being heard 4 city blocks away, that probably a good indicator your system is too loud. Nobody wants to hear loud pipes, Harley, Sportbike, or otherwise. If you are one of those idiots that sits at a light and hocks the throttle, or stunts, or performs other acts that bring attention to themselves... don't be shocked when the Police give you some attention. Stay low key, fly under the radar.