Do you feel agitated if only one earphone is working and the second one has become mute? We cannot throw them out or use them half-heartedly.
Most of the times, the reason will be the damage at the joint where cable and jack are attached. This may be because you put the mobile in jean's pocket and that causes friction between the jack and cable due to the twist, eventually, causing lose contacts.
Most of the earphones can be fixed. The trick is to cut the cable and solder them both again. But the branded earphones which have the fiber cables cannot be soldered.