I'm finding something simular which I hadn't noticed with IOS 5 so I'm assuming it's a new "feature" with IOS 6. I'm using a 4s upgraded to IOS 6.
When plugged into a docking media device even with the media device not turned on, the ringtone does not occur just the vibration. I happen to have an Apple dock connect to my stereo via composite av cable. With the stereo off I do not get any ringtone with my txt notification. When I plug into a standard charging cable I do receive my ringtone with my txt notification.
BTW, taking that same dock connecter and attaching it to a standalone charging connector and the ringtone is active again.
Maybe it always worked this way and I've just started to notice it.