i had this working on CM10 by unchecking this box, and using an app called dock audio switcher. It would detect the dock and automatically change the audio output through the usb. 

However, ive tried using this on CM10.1, and this app no longer works. I've also tried using the usb passive audio setting in the advanced menu, and this also no longer works. The odd thing is when i use the usb passive audio i get no sound whatsoever, but when i use the dock audio switcher i can hear a bump from the speakers, similar to when you first plug a audio cable into the phone jack. So it serems that its activating the audio to usb, but it just cant push it over.