The term 'Mountain Sound' can be broken down into two crossword solutions: ECHO (4 letters) and Mountain (7 letters). ECHO is a noun that refers to the repetition of sound caused by reflection of sound waves from a surface back to the listener. This phenomenon is often heard in mountainous regions, where sound waves are reflected off of the steep walls of the mountains. Mountain is a noun that refers to a large natural elevation of the Earth's surface rising abruptly from the surrounding land with a summit area that is usually much higher than the adjacent land.