notice on imac g4 flat panel

For some time now I keep getting the following notice on my g4 imac.

"The device plugged into the Apple speaker mini-jack cannot be recognized. Remove the plug from the jack. Then plug it back in and make sure it is fully inserted."

There is of course nothing plugged into the speaker jack.
I have tried everything including plugging the apple pro speakers in and out restarting with the speakers in and out all to no avail, any help would be appreciated.