Disappearing Volume Bar

I use an Apple Universal Dock to connect my iPod Touch to my stereo equipment. It's been working just fine for the last year or so.

The other day I plugged in the iPod into the Dock, and the Volume Bar disappeared and there was no sound out of the iPod! I connected the iPod directly to the stereo equipment and the Volume Bar was there and worked fine. It's only when I connect the iPod to the Dock that the Volume Bar disappears! What could be the problem? Thanks!