Headphones go bad because of wear and tear.  Depending on what type of headphones you have and how you use them, you may wrap up the cords, stick the buds in your ears, drag them on the floor, stuff them in your pockets, you get the idea.  The more use a set of headphones sees, the quicker they wear out and stop working.  We have all run into the day our headphones die when we need them the most, and every once in awhile it is not the headphones acting up, it is the unit itself. 

There is one simple way to check to see if headphones are bad or not on an iPod.  If you suspect you may be having sound issues due to the headphone jack, you simply want to plug another pair of headphones into it and see if you have the same issue.  If you do, the headphone jack may be going bad and you will need to get it repaired or replaced under warranty.  To test the headphones themselves, just plug them into something else to see if they work.  If not, then they are fault headphones and you will want to invest in a new pair.  Keep in mind the better quality the headphone, the longer they last if properly taken care of. 

Every once in awhile, the headphone jack of an iPod will work with one pair of headphones over another.  This is not a fault with the iPod itself but with the manufacturing line it came from.  However, under warranty, you should be able to get the iPod replaced for it.  Damaging the headphone jack is extremely rare as it takes a lot of impact to make it stop working.  If you use headphones, make sure to gently take them in and out as you store them and use the device.  This helps keep both the iPod and headphones lasting long into the future.