If you carry a passenger you should consider a good intercom.
I recommend Autocomm
Microphones are available for open face
helmets (as picture) and full face helmets.
The amplifier box can be situated under
the rear compartment (found by removing the seat) - I wedge it in
with a piece of foam to stop it vibrating.
It is really great when touring, to be able to chat about the scenery and other interesting things. It's so much better than travelling in silence and trying to remember all the things you wanted to mention when you reach the next stop. If you haven't had the joy of proper communication whilst riding then you owe it to yourself to try it out.