Why is my iPhone 4S no longer compatible with my Sony ICF-DS15iP dock since updating to iOS6.1?

Since updating to iOS 6.1 my iPhone 4S (16GB) is no longer compatible with the Sony audio dock. Obviously the FM radio still works fine, and when docked the phone still charges, but now when you play music the audio only comes out of the phone's integral speaker and not routed to the amplifed speaker in the dock.

Any ideas how this can be corrected, without rolling back to iOS 6.0 that is??