Theres a small voice changer with speakers, all you need to do is say...get some sticky velcro, velcro the changer to the back of the helmet where it's indented, then got glue the mic near the chin and the speakers....like always do something about those......
I know stormtroopers and halo red vs blue used this technique for their voices.
Maybe it wasnt red vs blue, but the old halo 1 TV commercials where spartans in red and blue armor sat around and talked about random stuff.