I'm not sure if its MS related or an auditory de-funk....

But I have it also. I describe it as lying in bed at night and hearing music play like my alarm clock radio volume is turned wayyyyy down but is on.

I did this a couple times in high school and set my alarm clock radio to go off (which it did) but the volume was turned down so low you would only hear it in the dead of silence.

And that's exactly how I describe what I have now.....only there is no alarm clock radio in my room to blame it on!