So i just settled myself down with a newest episode of The Office(US) and because its late i plugged my headphones into my mbp. I was expecting nothing more than normal listening levels but instead i was met with a really REALLY loud sound output. I'm on volume one with VLC turned right down. Quite suprising, but apple have probably recognised the fact that many people use the headphone jack for driving speakers, so have made the output quite substantial to ensure there is sufficient power. The upshot being, when you use headphone you've got to turn it right down. Check it out for yourself (I assume its normal).