I have the same issue
I have the same issue with my Galaxy S3. I just got a new set of Monster earphones. I only get sound out of one speaker. When I plug in the same headphones into my wife or son's Galaxy S3 it works fine. I plug in my ear buds that came with my phone they work fine on my phone. If I press down on the connection plugged into my phone with the Monster, I get sound out of both speakers. What gets me is that it works fine in 2 other Galaxy S3's and my original ear buds work on my phone but the Monster earphones won't work on my phone.