So, to add to the discussion, I tested this with Slacker last night. Played a few songs over my bluetooth speaker, then stopped Slacker and left it in the background. My battery was draining alarmingly fast, like nearly 10%/hour. I finally closed out of Slacker completely. The battery kept draining. Turned off bluetooth and it still drained at the same rate. I did NOT reboot the phone (because I knew that would resolve it). I put the phone on the charger. This morning, I got up, checked that it was at 100%, then took it off the charger and left it on the table when I went to work (I took another phone). When I got home today, it was at 55%. This is just with a wifi connection and nothing running after a full charge.
This tells me that it's not JUST Nokia Music, but perhaps any music app that may utilize the music hub framework. I suspect it's the music hub framework itself that's the issue. I have not test Xbox music yet. That's next. I strongly suspect that if I reboot my phone and use no music apps, my battery life will be great again. This is sad, though, because I shouldn't have to A) reboot my phone once/day or B) Stop using it for music.
Pretty disappointed overall and I'm wondering if other Windows phones have this issue, or if it's just the 920.