I'm now on my second, not quite so broken 6000 as my original newish one, and I have the same problem.... when I plug in the 2.5->3.5mm adaptor and my headphones I only get mono sound, and when I "dmesg | tail" it says
wm9712_update_jack_state: HP jack state changed (Jack=1,Mic=1)
when I unplug I get
wm9712_update_jack_state: HP jack state changed (Jack=0,Mic=0)
I have tried shorting out the headphone jack on either channel and watching the logs but don't see anything.
I have tried just plugging in the adaptor without the headphones too.
I can't believe two zauruses have the same fault!
Is the headphone circuitry detecting a very specific resistance?
I really really want to be able to use my Z as an in-car mp3 player with external USB hard drive, so stereo sound is vital!