Just a suggestion. Try recording your voice in a quiet room, mic not in the helmet, just holding it. And try different locations, like directly in front, to the side, slightly high, slightly low. But, all very close to you like it would be if it where in your helmet. Then see (listen to) what happens. Is your voice still muffled. If it is then its the mic. If not, then the best sound/location you get might help position it within your helmet. Then you'll know that's the best you can get from the helmet.