There's no way that I'm aware of that will allow you to send audio to both the speakers and headset simultaneously, but I'll be happy to bump this post back up in case anyone has some expertise. Unfortunately, I can't help with this kind of modification.
I recently bought a Samsung T220HD monitor. I love it, but I want to be able to have a headphone extension cable plugged in at all times. I know if you plug any headphone cable into the jack on the back of the monitor though it will keep sound from going to the speakers and will only play out the headphones. Do you know of anyway to either keep this from happening? OR (and this might be the only solution) do you know how to relocate the jack from the back of the monitor? I know this will require desoldering from the motherboard, but then I can add some cable lenth between the jack and the board perhaps???