In this video of a soccer riot in Turkey, the police are shown wearing riot helmets with some electronic contraption on the back: http://www.liveleak.com/view?i=859_1336904917#comment_page=2 . The helmets are clearly visible from about 1:25 to 1:40, and then later also.
What is that thing? A little calculator for figuring the current Euro? Brain control device?
Just a guess but I’d put my money on it being a radio transmitter/receiver.
It can give you a big advantage to stay informed an coordinated when trying to control a mob.
My WAG would also be a radio of some kind, though I’m at a loss as to why they would put it there. 
-XT
Well if someone was attacking you from the front, or more likely you are moving forward in a crowd, it’s less likely to be damaged in the attack from sticks, rocks and other flying objects etc…
It’s probably easier to build the speakers into the helmet than to wear headphones or earpiece that blocks hearing and could fall out when you put the helmet on, or afterward, in the middle of a riot; or if your action accidentally stretched the earphone cord…
You can probably fit more battery power in that than a shirt-pocket device.