The term 'flat sound' can be described by three different crossword solutions: HISS (4 letters), SSS (3 letters), and SSSS (4 letters). HISS is a sound made by a snake, which is often used to describe a sound that is low and drawn out. SSS is an onomatopoeia for a hissing sound, which is also often used to describe a sound that is low and drawn out. SSSS is an onomatopoeia for a sound that is low and drawn out, but with more emphasis than the sound of a HISS. All three of these terms are used to describe a sound that is low and drawn out, and can be used to describe the term 'flat sound'.