You do have a couple of options about how to adjust the volume of any track:
1) Click on the "bell" icon then the sound is muted (when the icon disappears, the track is muted).
2) You may quickly mute/unmute all tracks on the timeline by click on the main "bell" icon.
3) To adjust the volume, click on the media on the timeline and scroll down on the media settings on the right and slide the button of the "volume" or change it manually. Feel free to set up fade in/out as well.