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Phone going off or music player going off? Can't say I've experienced either on my S3 (though I did see the former once or twice on my S2)

I wonder if it's incorrectly detecting you've unplugged your headphones? Most android media players automatically pause music when you unplug your headphones, to avoid it blurting out loudly and embarrassingly over the phone speaker (this has happened to me several times in the past on Nokia handsets in the library, it's not fun). You can turn that function off in many media players, though if there's a fault with your headphone socket I'd get that looked at first.