I agree with the throat mics. If youre speakers are only 20mm than you could maybe mount two of them facing directly downward next to the Tvisor inside and have a throatmic system hooked up to it.
Now, some of the throat mics they have are gigantic, but I know for a fact they make smaller ones.
OR!!!!!
Hook up the Radioshack headset like is pictured in an above post and have the speaker VISIBLE under or inside one or both of the ear caps. I mean, in the real star wars universe they are using these mics and amps in a similar fashion and would have a similar problem. If you hooked up two small speakers on the outside of the helmet, facing forwards just on or in the ear caps, then they wouldn't look out of place and would give a good directional sound. Let me get a MS paint done and I'll show you.
Edit: Sorry, I forgot I was on TDH. Used to posting about customs on the Mercs forums. I corrected my theory in the bottom post. Included a way to hide them.
~Bio