This is a Low Volume 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 13,000 icons, all in the same style and quality.
The icon looks like a triangle with curved edges laying vertical but the point facing left is missing. It is replaced with a smaller rectangle shape with curved edges. At the base of the vertical triangle which is facing left is a small backwards C shape at the center.