i have a Toshiba running on vista under a year old. when i play anything using headphones the sound only comes out of the right side. i have used many different headsets and the jack does not appear to be loose.
First, open your volume controls. Look at all the levels on your volumes, and also check the Balance of each. Sometimes this can mistakenly be moved which would only play the music on one side or another. I would particularly look at your "Line out" or "Headphones" settings. If it still does not work, make sure the plug is securely seated into the headphone jack. If none of these fixes your problem, chances are the headphone jack is bad.