There are various reasons why a pair of earphones play audio-only out of one side. The major reason is that the audio jack wires have been bent so many times back and forth that have caused wiring short. If there is debris in the earphone, it could also cause the earphone not to work properly. Another reason could be the settings of your device. Go to settings >accessibility, then under hearing look at the mono audio checkbox, if that is checked, only one earphone will work. Earphones use fine stranded wire vanished in a slim layer of lacquer. The wire is then soldered at both ends to the plug and earphone. If the wire has a solder coat that is not attached directly to an anchor, it makes the wire brittle. Continuous flexing near the solder joint will make the wires individual strands to break off slowly over time. Finally, there will be just two or one strands attached by the joint, and the resistance of that channel shall become too high, resulting in the earphones cutting out.