You can adjust the audio volume of any audio or video clip in your presentation individually.
To enable volume adjustment for audio and video clips, click the Clip Volume button on the toolbar. See Timeline Options for details.
Every audio and video clip on the timeline contains a straight line that indicates its audio volume.
To adjust audio volume of an audio or video 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.