How much do earphone cost?
How come the left earphone doessn't sound as loud as the right earphone?
right--main music/beat left--base chorus. For iphone/pod/touch, you can change this in a setting called the equalizer
How much voltage do earphone need?
How much do Web-Cams cost usually on average and how much does Skype cost, charge i'm not that familiar with?
Webcams are typically around $30, and Skype is a free service.
What if your earphone is not connected to your mobile but still it is displaying that earphone is connected?
They were receiving instructions and timings. Also, because sound travels much slower than light, each performer would have had the music directly in their earphones to ensure they all stayed in time.