I had the same problem with my S3. It took awhile to show up, but eventually one of my earphone jacks stopped working.
I ordered this part from etrade supply:
OEM Samsung Galaxy S III SCH-I535 Loud Speaker Module ,Black
There are instructions online, that a very straightforward. unscrew all the screws from the back, GENTLY, pry the case apart, and replace the top part containing headphone jack and loudspeaker. One caveat that wasn't made clear is that it is a little tricky to actually plug the new piece into the phone. Don't use too much pressure, but it does plug in when you line it up properly. This step wasn't well explained. Now my phone headset works like new. It's only been a few weeks since I replaced, but it cost me about $35 for the part and a mini screwdriver tool including shipping and handling. If you have an eyeglass screwdriver you probably don't need to buy a multi-tool. I have an I535 but the part I ordered didn't list my actual model number and it still worked.