A plug assembly for a xylophone resonator includes a cover adapted to be mounted on the xylophone resonator to close an opening of the xylophone resonator, an annular disk adapted to be received inside the xylophone resonator and a threaded rod having a threaded bottom end extending through the cover to securely and threadingly connect to the annular disk and a threaded top end engaging with a knob. The threaded rod is selectively movable relative to the cover via the control of a controlling nut such that position of the annular disk inside the xylophone resonator is adjusted.