It seems as though there are probably a million things that effect the sound of an engine. I think most of it probably has to do with the type and size of exhaust, etc. There is the unmistakable fact, though, that a 4.3 liter v6 (which is a 350 with 2 fewer slugs) sounds nothing like a 350. Why? Even if you disconnected two wires on the 350 it wouldn't sound like the 4.3. I can't explain that. Probably something about the amount of expansion room the exhaust gasses have, the total volume of gas in the exhaust etc.
Cams are easier to explain. A car idles choppy because the overlap of the cam is allowing dirty exhaust to remain in the chamber, because the air velocity is not yet high enough to create the scavenging effects needed to move fuel/air into the chamber and suck the exhaust gasses out. Essentially, its running choppy because its re-burning dirty air.
Mufflers just redirect and cancel some if not most of the sound waves traveling down the exhaust. Packed mufflers cancel more sound than welded, in most cases. I prefer welded, some don't like the tinny timbre to them, but they last longer than packed and rap on their way down well.