The Valley of the Musicker was where Dorothy, Button Bright, and the Shaggy Man encountered the Musicker, Allegro da Capo.
Geography
The Valley of the Musicker is described as a pretty but empty valley with only one house, that of the Musicker Allegro da Capo. When one approached the lonely house they could hear the sounds of music coming from the Musicker, and that often drove visitors away. This valley was located in the same queer country as the land of the Scoodlers, Foxville, and Dunkiton. (The Road to Oz)