As Rulland said, I always fitted a mains contactor to the music system supply so its going to shut down the music audio and allow the evac signal to be heard, no ambiguity, no blame on you when their music system goes tits and fails to work.
If the music system is the same PA/Amps as the fire PA then thats fine (Never seen it in a club though??), done similar in a shopping centre as long as the fire signal can under all conditions over ride any other audio input and the fire signal volume is fixed then Bobs your uncle and Fanny aint a virgin.