You can adjust the audio volume of any audio clip in your presentation individually.
To enable volume adjustment for audio clips, click the Clip Volume button on the toolbar. See Timeline Options for details.
Every audio clip on the timeline contains a straight line that indicates its volume.
To adjust audio volume of a clip:
Mouse pointer will change its shape to a double-pointed arrow.
Drag the audio volume line with your mouse up or down to adjust the volume of a given clip. Current audio volume will be displayed in the tooltip.
To mute a clip, right-click on the clip and select the Mute option in the drop-down list.