I posted a similar question but didn't get the expected reply.
I asked several kiteschools instructors and they buy a few walkie-talkies, put them in a nylon (or Aquapack) bag and tie them to the helmet. They calim that the trainees can hear them perfectly. Search the web for Motorola, Cobra, Uniden and similar walkie talkies. Some of them are waterproof, but I wouldn't rely on it if they get submerged.
I thought about making a small hole in the bag to slip the Earbud through and then seal it with some epoxy.
A generic spec for the talkie shoul look like this:
Includes 2 2-Way Radios, 2 Earbud Microphones, Charger and Rechargeable Batteries
2 to 5 Mile Range about 10 Channels / some Privacy Codes
1 Year Warranty