Pretty much every method of moving the subvocal speach from the ’speaker’ to the listener, is going to involve some form of detectable communications means. Radios work well unless you need radio silence. Spred Spectrum can help, but is hardly undetectable. Optical links may seem like a near panacea, but if you can’t see the person you are trying to talk with, it’s not likely that an optical system will work. Even if you can, optical sensors very well may note the light system being used for communications. And of course no covert ops would even consider using a wire based ssytem. (Granted if you are right next to each other that may work, but how do you tell the sniper to start the mission from 200 feet away?)

That’s not to say that this won’t be used very heavily in the covert ops market. Just that it is not by definition undetectable. The real effort in nearly all covert ops is to get under the perception level of the opposition. This will help.

-Rusty