Robert_Charlton - 5:05 am on Feb 1, 2012 (gmt 0)
My first take as I read your post is that it's that something inside the headphone jack is not returning to full contact position when you remove the phone plugs.
What's been working like magic for me in all sorts of electro-mechanical situations like this is some "Super Contact Cleaner" I bought a few years ago at a local electronics parts store (sadly, a dying institution), not at Radio Shack. The brand I have is MG Chemicals, and the stuff is marked as extremely dangerous and very flammable. Somehow, it's different from ordinary contact cleaner I've used. Try not to inhale any of it when you spray. Use it outside if you can.
I'd spray it into the socket and then plug and unplug the headphone jack a bunch of times. I've also used it to fix situations in which Firewire or USB plugs/sockets aren't providing adequate signal, and for cleaning up PCI slots on motherboards when things aren't working right.
For me, it's become the modern equivalent of kicking something to make it work. ;)