iI had this working on CM10 by uncheckingun-checking this box, and using an app called dock audio switcherdock audio switcher. It would detect the dock and automatically change the audio output through the usbUSB.
However, iveI have tried using this on CM10.1, and this app no longer works. I've also tried using the usbUSB passive audio setting in the advanced menu, and this also no longer works.
The odd thing is when iI use the usbUSB passive audio iI get no sound whatsoever, but when iI use the dock audio switcher iI can hear a bump from the speakers, similar to when you first plug a audio cable into the phone jack.
So it seremsseems that its activating the audio to usbUSB, but it just cant push it over.