One earbud louder than the other?

I've tested my apple earphones with the remote and mic with my ipod and with computer and have listened to each earbud in different ears but no matter what song is playing the left earbud is always noticeably quieter than the other.
I've tried going on the support page where you can enter your serial number but I can't find it one the earphone box (this is my second pair which I only bought on the 11th of august).
What can I do?