There may be some kind of fancy digital sensor which can know if headphones are attached. Such things are not impossible.

For instance, on my new HP Win 7 desktop with an XFi card, I was futzing with the line input getting an old little Alesis parametric eq setup as a mic preamp for digital conferencing with an Audix OM5 mic. Sounds better than cheapie computer mics, and much more durable. I was having an intermittent adapter/wire between the preamp and the line input, and every time I'd unplug the plug from the 1/8" XFi stereo line input jack, Win 7 would pop up a warning that nothing is plugged to the Line Input and the input had been made inactive.

That seemed kinda overkill to me, that every time I unplugged the line input plug Win 7 would turn off the input, and I'd have to revisit the control panel dialog and turn it back on as the default input after I plugged a wire back in. That is the software holding my hand way too much. If I want the line input to always be the default input, then what the heck does Win 7 or the XFi card care if there is nothing plugged in? Annoying.

Anyway, that has nothing to do with your problem, except to demonstrate that sometimes computers/drivers will go to a whole lot of trouble to solve problems that don't exist, and sometimes make it worse in the process.

Have also read that some Apple Mac hardware is smart enough (or perhaps that should be dumb enough) to detect if you unplug a mic or a line input, and automatically reconfigure the audio driver accordingly. Maddening.

So maybe your friend's computer is smart enough to know in the software whether a plug is connected to the headphone jack, and then do software-controlled auto-routing.

However, for many many years on many stereo receivers, boomboxes, portable radios, etc-- The speakers get muted in a much simpler fashion when you plug in the headphones. The headphone jack just contains some switch contacts. If nothing is plugged into the jack, then the audio goes thru the jack switches into the speakers. When a plug is inserted in the jack, it opens the switches and interrupts audio to the speakers. No software wizardry required. Just a simple switch in the jack.

Whether or not the laptop speakers are controlled directly by jack switches, or whether the driver software controls this-- If the driver software controls it, then there has gotta be a jack switch in there to tell the computer whether something is plugged in. So maybe it is a bad switch in the 29 cent headphone jack either way.

If it is a bad jack on the mobo, once you find the exact inexpensive replacement part, it is just a tedious hassle to fix it yerself, or pay a local repairman maybe quite a lot of money to replace the cheap part for you. If its 6 months old and under warranty, maybe make HP fix it?

James Chandler Jr
http://www.errnum.com/