Why don't I have a custom title by now?!
It actually is supposed to sound like the SSG's firing sound, which works ok in vanilla, chocolate and prBoom+. ZDoom and GZDoom however mute this sound, perhaps on the basis that the Ultimate Doom IWAD is used. Just another side-effect of a "doom" mod using doom2 features which, I gotta admit, is very unusual and I don't remember it being done in other mods.
Another thing with the blunderbuss, it seems to be missing a fire sound.
That's how it's supposed to work, more or less. Picking the "repeating pistol" is assigned to slot 3 (it's essentially a chaingun, but it's de-hacked to trump the normal shotgun), while the "musket" gets assigned to slot 4, trumping the chaingun.
Also, a couple of the weapon slots are messed up. I pressed 4 thinking I would pull out the chaingun replacement, but it was the blunderbuss. So I pressed 3 thinking that the slots were swapped, but slot 3 was also the blunderbuss. I was like "WTF", so I pressed 3 again and then it switched to the chaingun.
If you cheat with IDKFA, you actually do get 2 weapons in the same slot (3), because the musket is actually the SSG, and you once again have a conflict between Doom and Doom II features, which is handled very differently accross ports. You can't do that in doom.exe or chocolate or prBoom+.
*shrug* will have to verify that visually, since I didn't touch anything regarding offsets in the PWADs. Chances are they simply did a poor job aligning them. I was surprised to see that the maps themselves were free of mapping errors, though.
Another oddity was the way the trees break, the offset for the animation is wrong or something. Shoot a tree with the crossbow/rocket launcher, you'll see what I mean.