For diagnosing any headphone issue, the first thing is to take it apart which can be done either with a little bit of force or with a screwdriver. However, for disposable headphones, do not use too much force or it will most likely break. If it is a simple issue with just the right headphone, then it is safe to assume that the problem is between the main cable and the right side of the wire, if it is an issue with the left side of the headphone then it is a problem between the main cable and the remaining wire.
Once the headphones have are opened, sometimes the problem is right there, and the wires have come away from the solder that attaches them to the speakers. If this is not the case, then assume it is something wrong with the wires further down. Every centimeter of the wire needs checking for any discrepancies. There is also such thing as wire resistance if it is sufficiently resistant then there is an issue there also.
There are two simple reasons as to why headphones may stop working. The first is that the jack has stopped working the second is that the speaker has stopped working. To diagnose if it is the jack, the wires around that area needs wiggling around a bit, a cracking sound would is heard at this point, or the music will go off completely if it is a problem with the jack. If it is an issue with the speaker, then the same steps need to be repeated for the wires leading to the speaker and again if a crackling sound is heard, then it is this area.
There are also some simple reasons as to why they may not be working.
Is it plugged in correctly?
May sound simple enough, but the plug in the entrance port needs to be double checked to make sure that it is connected to the audio device properly. The easy way to tell if this is the issue is to test the plug itself, or sometimes sound may be heard through only one headphone. If this is the case, then something could be obstructing the port from the inside.
Is the audio device itself faulty?
The issue may not even be with the headphones it is possible it is a problem with the audio device. The sound port may have the fault. The way to tell if this is the problem, plug the headphones in question into a different device and if they work then it is not a fault with the headphones.
Headphone audio cable problems
Once the plastic covering the wires leading to the headphones is broken, it is easy for the wires to become damaged. If the cable is damaged it could be anything from them cut, frayed, ripped, and sliced or only hanging loose.
Headphone jack problems
The jack on the headphones may have been broken or bent; this is unfortunately quickly done as headphones can be sat on, stood on or just only be bent at the bottom of a bag or pocket.
Internal sound issues
If the audio problem is something to do with the headphones is it possibly from the audio driver. The way to diagnose this is if you have sound coming out from only one side of the ear cup if the sound is coming out too quiet if the audio is coming out with scratchy interference, which again is fixed with a simple cable fix.
It is easy to repairs most of these issues. To change the jack on a set of headphones the old jack needs to be removed and a new one with the same number of wires connected to replace it. The same with the speakers where the old set is removed and a new set replaces it. Finally, the attachments can be added with super glue or any other strong glue, but it is better to be able to solder the parts, as this is more permanent for most solutions of cable repairs.