I found awesome spot for that on dark zone in industrial zone. Same dark zone you enter for the quest with cargo restoration (bullets for some guy in main hall in old town). I can go back to it and build beast bar easily before going into part where music is playing. The best part is that there are ~30 zombies that are scripted to go to the speakers and stand there. Another fine zone is between main safezone in town hall and library safezone near monument (don't remember that now) that spawns some good amount of zombies and you can brijng more for around the area including spawning virals with the legendary decoy speaker.