I built a second prototype with a new helmet and a Bluetooth headset with an integrated microphone. Now the user do not have to take care of any wires while connecting his mobile phone to the helmet.
The first tests have shown, that the music is loud enough to listen to it while riding and it is possible to hear the environment. But if the speed is higher than 20km/h the headphones and the headwind are generating an annoying noise.
The same happens with calls. And due to the fact, that the microphone does not has a cover at all, the caller cannot understand a word if the bicycle goes faster than 15km/h. But maybe this problem can be solved by cover the microphone with a small piece of foam (-> next task).
Due to the fact, that the headphones are just taped to the helmet, the wind lifts them while speeding up. So they have to be fixed to it in another, more stable way (-> next task)
And I still go no answer from the Buhel company regarding their bone-conduction microphone.