It seems that at least the official campaign(s) are addons so you could have a shared script and sound folder in the root of that pbo.
Just de-pbo BIS missions.pbo from the game's addon folder and you'll see how they did it; inside the missions.pbo they have a folder
campaign so at least in theory (and I don't see it wouldn't work) you could store that shared sound and scipt folders in the root of the pbo.
As in your campaign pbo would include:
- campaign folder
- sounds folder
- scripts folder
You could even make a config in the sounds folder defining the sounds so you wouldn't have to add them in each mission's description.ext..