When you open slave windows, the music in the main window turns off. After turning it on again, the music starts playing in all slaves as soon as a boss appears - and there's no way to turn it off (except for killing all slaves) because you can't access the slave controls.
When you open slave windows, the music in the main window turns off. After turning it on again, the music starts playing in all slaves as soon as a boss appears - and there's no way to turn it off (except for killing all slaves) because you can't access the slave controls.