I might list this as a bug, but am unsure.
I have noticed that when a NPC wearing a helmet is speaking makes a muffeld voice, while putting on alot of emphasis on P's and S's instead of making a slight radio effect on the voice. This can get quite irritating. Ive never notice this whilst playing FNV on my Xbox 360.

IF this is a bug, please patch it or, tell me if there is a patch for it. (Ive also noticed this on Fallout 3 (PC)),

I remember the radio voice effect in Fallout 3 and noticed it's missing from New Vegas (I get the slightly muffled effect too), I'm not sure if it's intentional or not.