Radioshack.com and instore has this portable stereo speakers that fold and are thin.
http://www.radioshack.com/sm-portable-fold...pi-2103838.html
This will run off a battery or a ac adapter plug.
Because it uses a stereo 3.5mm jack, it can be converted with a adapter to have a mic be plugged in. The the sound from the mic should be directly transfered to the speakers.
This may not fit inside a helmet, but can fit behind the chest piece of almost any suit of armor.
THe total cost will be around 30 dollars if you dont have the mic or adapter.
Sorry I couldnt post a pic of the adapter, I have one, but I dont know the exact name of it.
EDIT:
I found a cheaper speaker with good sound quality according to reviews. 16$ from amazon.
http://www.amazon.com/Cyber-Acoustics-Port...1093&sr=8-3
http://www.radioshack.com/sm-portable-fold...pi-2103838.html

This will run off a battery or a ac adapter plug.
Because it uses a stereo 3.5mm jack, it can be converted with a adapter to have a mic be plugged in. The the sound from the mic should be directly transfered to the speakers.
This may not fit inside a helmet, but can fit behind the chest piece of almost any suit of armor.
THe total cost will be around 30 dollars if you dont have the mic or adapter.
Sorry I couldnt post a pic of the adapter, I have one, but I dont know the exact name of it.
EDIT:
I found a cheaper speaker with good sound quality according to reviews. 16$ from amazon.
http://www.amazon.com/Cyber-Acoustics-Port...1093&sr=8-3