Hi,
The manual says:
Volume
This smart control is only shown if you select All in the Smart Controls pop-up menu in the Inspector.
Allows you to edit the volume of the segment. To mute a segment, turn the volume down. This also works on a selection of segments.
But you can also use the Mute KeyCommand (Shift+M) and Unmute (Shift+U) or the same menu function: Edit > Mute; Edit > Unmute.