Theres probably a short circuit in there somewhere. The quartz signal from your mic must be picking up signals from your wires that are supposed to to go to the headphones. (OR UR FRNDS HEADSET HAS A NOISE CANCELLING SYSTEM[THE OLD ONE!] )
Repairs are easy once youve noticed the problems. The electric wire carrying the quartz signal is usually purple or green. As for the wire(s) going to the headphones that carries the signals for sound it's hard to guess.