Any question answered...

Confused:
I have used the same computer at work for the past 2 years and now when I plug headphones in to eiother the front or rear headphone jack it continues to play msuic from the onboard speakers while also playing music through the headphones. What's going on?

Same headphones? Normally it relies on the plug either pushing some contacts together or bridging them to send a signal to the OS. Either something's broken that mechanism or some kind of OS update or software install has interfered with how that signal is handled.

I tried 3 pairs of headphones. Same problem with each. No change of audio drivers, or OS version (still on XP here) as far as I am aware.

This is weird. When I mute the sound by clicking on the speaker symbol in the Notification Area of the XP task bar I can still get audio out of headphones, but not from the computer's onboard speakers.

Problem solved, although still very confused.