Who invented earphone?
The input to the earphones is a a analog signal i.e., electrically generated analog signal.
How was the earphone used when they were invented?
If you want it because you want good bass get Skullcandy skull crushers. It all depends what you are going to use it for.
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
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.
How to put on earphone?
Yes I currently have some which came with it in an Irish chain store called Xtra-Vision.