Sorry TW the volume of the music would decrease very slightly due to the lack of the surface of the door to provide a reflective surface for echos back to the listener and i'm not sure but this may also happen for the light as well.
perception vs actual; If you're listening to music in a Darkened room and you switch a light on, the music seems to go quieter but it doesn't really. Does this help your question?
I think what NJ means is that when the door opens some of the light and sound spills out into the hall, does this reduce the volume of light and sound in the room? I guess it must come from somewhere, but if it happens just turn the volume up !