Let's push the reset button here. As per usual on the forum, we routinely digress. Not that I'm complaining, I do it too. But my original question was why does old school sound so much better than new school?
I think we got off topic a little because the answer had already been explained prior to the the diversion.
Your BIGGEST difference is in the cam timing and how engines make power today vs yesterday.
Roller lifters allow for much faster opening and closing of valves. The resulting smoother idle with more vacuum at low engine speeds sure doesn't sound nearly as "cool" or "racy"........but it does work WAY BETTER in that it doesn't leave the bottom end of RPM range DECIMATED for power in the way a flat 'tappet" cam did with it's far less aggressive open and close speed. The old design overlapped an intake and exhaust valve opening and closing at low engine speeds so severely that it created that "racy" sounding STUMBLE in the exhaust note.
The other big difference is not really an exhaust sound at all.
Anyone old enough to remember "flipping" an air cleaner lid on a carburated car with no changes under the hood to get a louder acceleration already knows this......but for anyone too young to remember doing that here is something else that is different today vs the old school carb. INTAKE NOISE......it can be HORRENDOUS LOUD as an engine ingests air to mix with the fuel it burns. An open element air cleaner with a 4 barrel Carb sounds louder and different than modern air boxes and throttle bodies.
The exhaust under the car?
Hey, remove the cats and add long tube headers ....... virtually every older car will have this set up because most didn't have Cats and it was super cheap and completely legal to do. Modern cars? People do it but it's not legal to remove cats completely and long tubes for the modern muscle car.......pretty expensive so they are less commonly installed.
It's just my opinion but in these few points ......I think is where most of the difference you hear is found.
It's also hard to compare "apples" to "apples".......engine displacement matters too and today it's rare to find exact an comparable between new and old. Yet displacement differences will change things too even when all other things are equal.