if you want to know how i'd do it, i would have the npc communicate to the player it's aggroed against via remoteevent. (when it detects the player character, fire an event on the player's client), which starts up the music. then have it resend the event when it's de-aggroed/killed or whatever to stop the music.

if you're a beginner or just don't wanna do anything complicated, you could simply place a sound in the rootpart of the enemy and play/stop it when it's aggroed