I understand the frustration! Been through it myself.
The way my front headphone jack works is that it will cut off my speakers which are connected at the back. From your post #11, that returned your sound to speakers when you removed headphones from front jack (and turned on speakers
). From post #13, headphones worked ok on laptop. From post #15, two sets of phones tried, didn't work either on PC, but do on other computers. My link (Guide to the Audigy 2 ZS) in post #17 gives you step-by-step information on installing your sound card. Doesn't hurt to print it out.
If your phones have been plugged in for a year, the metal on contacts may have oxidized to the point there is no electrical connection. A good contact cleaner for electrical contacts that leaves no residue can be tried. I usually spray a little inside plug, insert/remove phone plug gently several times, respray and repeat. A gentle twist of plug may help cleaner to do its work. Doesn't hurt to use a clean clothe to clean plug after each insertion. Since it seems phone/speaker switching is done by software, all this may be to no avail.
A quick way to check plug, is to insert another sound card and see if plug and speakers work. Sound cards do go bad. Although I don't think this is the case right now. Just remember to discharge your body against case before poking around inside to remove all static electricity. And don't forget to make sure PC is unplugged and card(s) are properly seated! Same for connectors.
Win XP also has a good Sound troubleshooting method. Through Control Panel>Sounds and Audio Devices. Or, Start>Help and Support>Pick a Help topic>Games, sound & video problems. Good bet is you've been there, done that.
Ron