I thought I had a similar problem when I had my first S3, but as Mark says it was due to the settings on the phone. Someone here informed me that there was some sort of equaliser that you can adjust, but you have to be on a call to do it. (The setting stays even after you finish the call.) Unfortunately I can't remember how.
In my experience the S3 has so many features and is so customisable that often you think there's something wrong when it's simply a case of the settings you have chosen.