Everything about the weapons in the original UC feels so badass. I think the sound was a big part of that, and for whatever reason I don't feel the same way about UC2 and what I have played of 3.
Is the sound the gun makes really that important.
Well yeah. For comparison you don't want a shotgun to sound muffled you want it to sound like it has authority, the sound associated with a shotgun. For a real life comparison the sound of the Nazi MG42 in WWII scared the US infantry. So yes sound is important.