I was trying to play music throught my iPod dock with my new iPod (5th gen). The sound only comes throught the iPod and not the stereo.
Sounds (sic) like there is either an incompatibility between the old dock and the new iPod, or the iPod docking port is defective. Take them both down to your local Apple store for the "gurus" to look at and advise you.
Where I live, there is no Apple store. I think I would just be because the dock I for iPod 4th Gen.
Go to the Apple web site iPod support page. You should be able to post a query there about this issue.