This window is accessed by clicking the up arrow to the right of the Voice button on the bottom button bar. Alternately, you can open it from the top menu, Comm → Nearby Voice.
The window shows your name and icon at the top. Below this is a drop down for Voice Morphing. You can get a Preview, subscribe to it, or enable it. More information on voice morphing is available on the SL Wiki.
Then a list of nearby speakers: an icon and their name, a slider with which you can regulate their speaking volume, and a symbol indicating where they are currently speaking.
At the bottom is a slider which controls the volume of your own voice when speaking.