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The armor parts only change your look, not your defense.
It seems as if the armor does something other than dress up your character, but I think it is just to compare yourself to the armored drudge/ soldiers in the campaign. Armor with armor upgrades would be very OP, anyways.
Don't bother. C2's mic system is extremely glitchy and very few randoms use mics. If you want to talk with other GameFAQs players, Skype would be a better option. (You can use the mic with Skype, but even that doesn't always work seamlessly.