I don't believe that you can fix the problem, likely a piece of dirt got into the volume control and is blocking the contacts however I don't believe the volume controls can be opened without destroying them all together.
It may still be possible to get them replaced by Apple, if the ipod you are using them with is still under warrantee (all new ipods come with a 1 year warrantee which will cover you'r ipod and the in-ear phones you'r using it with, however can be extended to two years by purchasing the applecare program) to check if the ipod is still under warrantee input you'r ipods serial number into this webpage https://selfsolve.apple.com/GetWarranty.do
Hope this solves the problem!