My battle song, would be a playlist, because I have no intent of being dragged down by simple rage zombies in the time it takes one song to play. For that matter, i would never get stuck in a one exit bunker in a zombie apocalypse. In my region, mobility is your friend. I would set up multiple home-bases about 6 hours walk from each-other in the woods where there won't be many people or zombies, that are all basically hidden tree-houses that can only be reached by ladder. Hide the ladder in the underbrush nearby, in fact hide 2 to be safe, and pull it into the tree at night while I sleep. Then, me and my friends will just keep moving. Raid civilization for food, and retreat to the safety of our tree-houses at night.