It sounds to me like you have some kind of sound profile/management app installed that's causing this problem.
You could try booting into Safe mode and see if it behaves properly.
Or do another Factory Reset, and before going through the device setup (just skip all of the Google account, etc) check to see if this is working correctly. If it is, then something that's being automatically installed when you establish your Google account is mucking up the works. If it isn't then the only thing I can think of would be to reload the stock firmware using the Kies software on your PC.
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