IPhone 3GS: suddenly music only on the left

hi. i can help you. begin by cleaning out the little hole your ear buds plug into. Carefully clean the jack hole with a needle or a tough pick. It allowed me to remove a lot of cotton like dust. if that doesnt solve it go into your phone menu under audio settings and see if you have adjusted the balance to cause your problem.
also, You can try removing any debris that might be in the headphone jack by plugging and unplugging the headphones a few times.
If the issue is not caused by debris in the headphone jack, request repair/replacement of the Apple Earphones with Mic online here: https://selfsolve.apple.com/GetWarranty.do

You can also request service by making a reservation with a Genius here: http://www.apple.com/retail/geniusbar/or call AppleCare U.S. iPhone technical support at: 1-800-MY-IPHONE (1-800-694-7466)

Usually it's broken headphone jack module. You can try to replace the jack yourself, the part is on Ebay, there are iPhone repair guides that show you how to do it. There is a service that offeres repairs for the iPhone 2G, 3G and 3GSand provides a one year warranty on all new replacement parts. iPhone Repair NYC

Try this, insert the earphones slightly into the hole and then slowly inch it all the way in and out of the jack. With any luck you will get stereo sound poping in and out again whenever the plug is slid to a specific location.

If this is the case, everything is fine. It is just the headphones you bought. There is a slight difference in where the contacts line up between headphones of different types. The iPhone/iPod headsets are a little shallower and small, bargain bin earphones are a bit long. There is some room for play (two sets of aluminum tabs that squeeze against the metal), but sometimes they're too different to work perfectly.