To my knowledge this happens on Dungeon Run, Monster Hunt, Rumble Run, and Dalaran Heist: if you lose to the final boss, the first boss of your next run will play the final boss music.
To my knowledge this happens on Dungeon Run, Monster Hunt, Rumble Run, and Dalaran Heist: if you lose to the final boss, the first boss of your next run will play the final boss music.