I'm trying to remember a book set in an alternate England with a walled-off section that contains undead and other magical phenomenons, which can be controlled and dispelled by playing special musical bells.
The owner of these bells is the main character's father, and is responsible for maintaining peace between the walled-off area and the rest of England. When he is captured by a powerful undead, the main character goes into the walled-off land to rescue him using her own set of bells.
I believe this is the "Old Kingdom" series by Garth Nix. I haven't read it myself, but my sister loved them growing up. Always told me about it, and how the bells can also be used for opening gates into some kind of afterlife/other world.
First book is Sabriel (1995). From Wikipedia:
When the current Abhorsen is overcome by one such evil and beyond the Seventh Gate, he sends his bells (the primary tools of a necromancer and used in various ways to control the Dead) and sword to his daughter Sabriel via an undead messenger bound and under his control. Sabriel is at an Ancelstierre school for girls to remain out of reach of those who might try to strike at her father through her and end the Abhorsen bloodline. The bound undead is also attempting to speak, but she must enter death in order to make out the words. She is instructed by her father (speaking through the undead messenger in Death) to return to the Old Kingdom to take on the role of Abhorsen and stop Kerrigor's return to Life. While in Death obtaining her father's guidance, she narrowly avoids a fatal altercation with a Lesser Dead.
