Is your feature request related to a problem? Please describe.
I hate how Battle themes play at any time as long as a mob is within distance of you, instead of initiating when the mob has aggro on you. It causes some major issues, like mobs with different aggro ranges not working with battle music or battle music playing when nothing is attacking you. (Caves have this problem especially bad, as battle music plays for anything that is within 16 or so blocks from you even through walls)

Describe the solution you'd like
Change the Battle event to be based on whether or not an enemy is aggro'd on you. This would dynamically change the range at which the song would play depending on the specific enemy's aggro range, as well as fixing battle music playing when nothing is fighting you.

Either that, or add a separate event called Aggression or something where the event will play as long as the specified enemy (or if no enemy is specified, any enemy) has their aggression set to you.

Describe alternatives you've considered
No alternatives available for use at the moment, I just had to disable battle themes.

Great idea! Current attacked and horde events also didn't allow to create real battle music, because it stops after a few second you are not attack/attacked.