Dude, I've shot these cannons, multiple times. What you hear is a shotgun shell. The sound coming out of the tube of the cannon amplifies it. Trust me.
Even if it weren't a shotgun shell, these are not front loading civil war cannons. They are designed to be breech (the rear for you non-artillery types) loaded. The shell that would be fired is basically a giant cartridge, similar in design to what you would shoot from a rifle. There is no loading of powder, even if I was to fire standard ammo and take out the visiting band.
You want to believe that it is not as loud as it used to be because of some urban legend about less powder, be my guest. I'm telling you that the positioning of the cannon is the culprit, plain and simple. Maybe you heard it better when you were sitting in the student section and right next to the damn thing. Maybe it's a case of fond memories outweighing simple facts. Dunno. But I'm telling you how the thing works and has always worked. Not conjecture, not speculation, but actual first hand knowledge.