I would recommend bypassing the external speakers on the LCD and buying a good set of external speakers and hooking them up to the speaker port in the back of the PM. When you want to use headphones, you can hookup your headphones to the headphone jack on the speakers, OR hook up the headphones to the front of the PM and turn the speaker volume all the way down.
An even better option if you are into music production at all is a Firewire or USB based audio interface. Ideally it would have a couple mic inputs, two or more line inputs, line outputs (which you could send to your external speakers), and a headphone jack with separate headphone volume control. I happen to have just such an item (an M-Audio USB Duo) and it is wonderful. It sits right on top of my monitor and I can adjust the headphone volume easily without moving or using a mouse on the computer. I also keep my external speakers powered off most of the time unless I don't feel like using my headphones. I actually prefer the phones most of the time (I have the Sony MDR-7506s which I think are amongst the best headphones I have ever heard).