Since you haven't received a response I'll throw out a couple of ideas.
I've heard the same type of thing when headphones have gone bad and grounded a hot wire on one side, obviously not the problem with your setup. It's possible the software involved is sensing the headphone impedance and changing output to something other than stereo.
Things to check:
-If you're listening using iTunes, under Preferences/Playback, make sure 'Sound Enhancer' is unchecked.
-In the Utilities folder, open Audio/MIDI setup, check the Configure Speakers button and make sure the multichannel setup is set to stereo, and set to left-1, right-2, do this with the phones plugged in.
WARNING- avoid clicking on the 'left front/right front' buttons unless the sound is turned way down, you will get a blast of white noise.
-Instead of inserting the jack all the way insert it partially while playing one side only, when that side comes on move the balance slider to the other side and see if you're getting sound. (doubtful, no real reason it should work this way.)
The jack for the external speaker and the jack for the headphones are identical, correct?
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