That's so hilariously misguided, it's like some student's experimental pet project. It gives zero visual hint as to how to use it (i.e. ‘affordances’). From the looks, I'd guess that it's a horizontal slider with the range of like fifty pixels. If this was any kind of an established control, you'd surely just use the speaker icon itself as the ‘handler’ to grab—the fact that they needed the separate green dot says enough about whether it was a good idea. How it passed any kind of review at MS is a baffling mystery.