I had the same exact problem. It turned out the audio cable that connected my XM Unit to my laptop was bad. I replaced the cable and everything worked fine.
I don't remember making any changes recently, but I very well could have unknowingly changed a setting that has caused this problem.
Playing mp3s and normal audio files play fine out of all the speakers in my car, but when I play XM through the line in on my sound card it only plays out of the front speakers. I'm using Envy24 Family Audio Controller WDM. I've tried everything I can think of, even reinstalling all drivers, but I still have this problem.
Any help is appreciated.