"Flatter sound" refers to a sound that is balanced and even, without any exaggerations or distortions in certain frequencies. It means the sound is natural and true to the original source without any enhancements or alterations. Full definition
The weak point of most laptops is the speakers — a disappointing, flat sound which lacks the warmth to make listening through it an enjoyable experience. (thenextweb.com)
We have the opposite problem with sizing And your studded flats sound awesome! (modlychic.com)
You picked out a fabulous dress and heels to wear the night before but when you wake up, a pair of comfortable pants and flats sound much more appealing. (theeverygirl.com)