The term 'Sound Bounce-Back' is related to the crossword solution 'ECHO' (4 letters). Sound Bounce-Back is a phenomenon in which sound waves are reflected off of a surface and return to the source. This is known as an ECHO, and it is a common occurrence in nature. For example, when someone shouts in a canyon, the sound waves will bounce off the walls and return to the source. This phenomenon can also be used in audio engineering, where sound waves are bounced off of surfaces to create a desired effect.