I have the exact same issue, the audio jack on my desktop speakers works fine but none of my three sets of headphones work properly. Having looked closely at the jacks the only difference I can see is the one my speakers is extremely fractionally longer (less than 0.5mm - I'd estimate about 0.2mm) so I assume there is a contact at the bottom of the headphone port which our headphones can only reach when we really force them in and over a short space of time the jack moves enough that the contact is no longer made, turning the sound off.
