I have successfully paired my Samsung Galaxy S3 with '12 sonic, however, when listening to music it only comes through the front driver side speaker. Is anyone else having this problem, or know something I can try?
This happened to me as well (Droid Razr). I was taking it to the dealer to have it looked at and it magically fixed itself. My best guess is the radio got an update (bluetooth) through Onstar. Is Onstar still turned on? If not you may have to take it to the dealer. If it's on you can call them and have them check.
Have you had your MyLink update done yet? If not get it done first. Either way, mention it when you bring it in - likely your still covered under warranty. Radios do NOT auto update through Bluetooth/OnStar. Just an FYI.