If your iPod earbud headphones are defective, or even if you step on them, or they get caught in the garbage disposal, or your dog chews them up -- Apple will replace them for free as long as your iPod is still under warrantee.
You need to visit the iPod Service Request page and click through to the service request form. You will need to enter your iPod serial number and your Apple ID login. If you don't have an Apple login you can create an account and come back to the iPod service page to make your request.
The Apple iPod earbud replacement policy is stated in item number five on the iPod service request page.