This is a Adjust Filled icon. We've made it in iOS style, first introduced in iOS version 7 and supported in all later releases up until now (at least iOS 11). This style is based on thin two-pixel lines and is optimized for 50x50 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 12,500 icons, all in the same style and quality.
It's an image of three vertical analog equalizer sliders arranged side by side. The slider knobs are all in different positions vertically to indicate different selection of base, mid, and treble. It is three vertical lines each with one rectangular knob.