Real bug report here, I think.
When using the car dock, audio does not come out through USB, despite having that option checked under Device Options. I double-checked settings, unchecked, rebooted, checked option, rebooted, docked, sound came out through the built-in speaker.
To be certain I did a full wipe, flashed back to stock, docked, sound came out through USB as expected. Performed another full wipe, flashed to CM10.1 latest nightly, docked, sound came out through USB as expected. One more full wipe, flashed to latest RB nightly (20130130), checked option, docked...sound came out through speaker.
If anyone has any other ideas on what to do/try, I'll gladly try it.