You can adjust a clip's opacity and volume over time using the timeline graphs or in the Controls panel.
The Editor sequence is not intended for complex animations, for which you should use Composite shots.
Using the timeline graph
The timeline graphs are visual representations of volume and opacity over time. Each clip has its own graph.
By default clips start with 100% opacity (fully visible) and volume set to 0dB (full volume).
Clicking and dragging on the graph line adjusts the opacity and volume levels.
Keyframing the graph
To change the graph over time, hold down the Ctrl key and click on the line to add a new keyframe.
If you have at least 2 keyframes, they can be dragged up and down to create changes in the graph over time.
Using the Controls panel
The volume and opacity is also represented in the Controls panel. This is useful for more accurate adjustments.
If a video clip is selected the video properties will be displayed, including opacity. If an audio clip is selected the audio properties will be displayed, including volume.
Keyframing properties in the Controls panel
To change volume or opacity over time first click the grey circle buttons to the left of the properties. These will turn green and activate keyframing.
Once keyframing is activated, changing the property will add a new keyframe to the playhead's current frame.
RECAP Opacity can be used for dissolve transitions, while volume can be used to mix a multi-track soundtrack.