I personally use an
AKER MR2300 in all of my costumes. If you get it directly from the manufacturer they also do the TK TrooperTalk which adds in radio static as a squelch circuit like the Stormtroopers in Star Wars have for their comm links. It's a cool kit and 501st approvable so if you're looking at ever doing a Stormtrooper or Clone then you can double dip.
I just use mine for ambient music for my costumes and an in helmet microphone so that I don't have to yell through plastic/foam/metal to be heard at a convention.