Just to cover all the bases:
-- are you sure the plug is inserted all the way in? Sometimes certain headphone plugs are shaped in a fashion that prevents them from being inserted into the headphone port on a Powerbook all the way.
-- is there anything stuck in the headphone port, possibly a piece of headphone jack that broke off many moons ago/?
-- in the Sound preference pane of the System Preferences, under "Output," is the balance in the middle?
-- sometimes the headphone plug and/or the jack itself become dirty over time without use and don't conduct very well anymore. Twist the plug to and fro while it's inserted in the headphone jack and see if you hear any difference, perhaps left and right channels cutting in/out and/or crackling/staticky audio.
