Whispering Mountain — Joan Aiken

In the small town of Pennygaff, where Owen has been sent to live after his mother's death, a legendary golden harp has been found. Knowing of the prophesy of the Harp of Teirtu, Owen must prevent the magic harp from falling into the evil clutches of its reputed owner, the sinister and diabolical Lord Mayln. But it won't be easy. Owen and his friend Arabis are plunged into a hair-raising adventure of intrigue, kidnapping, exotic underground worlds, savage beasts...even murder.

For only too late will Owen learn that Lord Mayln will stop at nothing to have the golden harp.