sound problem iMac

[Tom Dempsey]

I have a problem with the sound on my iMac 17" flat panel 1Ghz Super 
Drive OS10.2.3.
I got this machine about 6 weeks ago.
The Apple Pro Speakers had worked nicely and still do. My problem is 
that one day last week suddenly only one speaker worked when I played a 
CD.

I was able to fix this in the Sound Preference Output Panel by 
centering the stereo balance slider. For some reason it was way over to 
the right. The CD played OK.

The next day the same problem again but this time Sound Preference 
Output Panel had no stereo balance slider. It was gone.

Upon restart the chimes are in stereo (or at least sound comes from 
both speakers), yet everything else including modem dialling and alert 
sounds come from one speaker. My headphones are work in stereo.

I reset the PRAM and the firmware. I reinstalled System software 
starting with 10.2.3 and then updated to 10.2.4 and then 10.2.5. Still 
there is no stereo slider in the Sound Preference Output Panel. The 
"device for sound output" is set for "built in audio controller". But 
under "settings for selected device" it says "there are no settings 
available for the selected device".

I have yet to find anybody else with this problem.
Can anyone propose a solution, please?
TIA
Tom in Skopelos Greece