Turned the looping background music option off and still getting the same results. And it seems to be easily repeatable.
1. Attack an NPC and the combat music starts.
2. Kill the NPC and the music stops.
3. Move the input focus to another window (web browser for instance) and the combat music starts again.
4. Go back to the Planeshift window, do something that has another sound effect (use magic for example) and the combat music stops.
5. Go back to the point 3.
This started after the looping background music option was added (should we blame
stfrn?

)