Iphone 5C earpiece jack problem

I bought a new iphone two days ago. i sync it to my computer only to realize that my songs can only be heard from one side of the earpiece. i tried using several other earpiece but the results were the same. so this morning i went to the apple store to get my phone "repaired" and by repaired, all they did was restoring the phone and tadaa, it work fine. Then i reach home, sync the phone and boom, same problem. Dont know if this is a software issue or whatsoever. really need to get this fixed. HELP pls.