One of the audio channels was not being transmitted by the headphone jack.
The repair took me less than 30 minutes. This was my first time repairing this kind of Ipod. The other kind was a nano, and I will say for sure that this one was MUCH easier than the nano. You can do it.
As others have said, the hardest part is getting the metal case off. Your best friend in this is the putty knife. I got it started with the plastic opening tools, but I relied on the putty knife to work it open. The hardware that holds the ribbons in place can be a minor problem, I could barely see the mechanism and it was stuck securely and I was afraid I might break it, even with the spudger. My advice: be careful, use lots of light, and have a magnifying glass handy. Also taking a "before" picture of the hardware inside will help you, if you start wondering if you did something wrong (you can probably find these online). The jacks went out and in with no difficulty, but it was a pain to get the ribbon back in, the two halves of the device were so close together that it made it difficult to grab the one ribbon and slide it back into place to be locked. I could have used a pair of rubber tipped pliers or something to help there, I got it in, but it was the second most difficult part for me. Also, when the little line on the ribbon is flush with the ribbon holder, it is now "in place" and you can lock it. One final bit of advice: when prying open the case, the metal backing may become slightly deformed, I gently sqeezed the metal backing of the ipod so the when I closed it, it would have a nice tight fit. Looks perfect, works perfect.
iPod Classic (Thin) Headphone Jack & Hold Switch
1.5" Thin Putty Knife