The crossword solution for the term 'ADDITION TO THE MIX' is REVERB, which is a type of audio effect that creates a reverberation or echo. REVERB is commonly used in music production to create a sense of space and depth, as well as to add texture and character to a track. It can also be used to simulate different acoustic environments, such as a concert hall or a small room. REVERB is an essential tool for any producer or engineer looking to create a unique sound.