See the quick start guide: Working with video.
To perform a fade, a simple way is to use the Set Alpha shader filter, provided as part of the user library.
- Add the video file.
- Add the filter.
- Connect one to another.
- Connect the filter output to the viewport.
- Add an automation to control the opacity.
Here is a complete example:
The core idea is to extract a parameter from the sound, and use it to modify a parameter of a video filter.
The Envelope process can be used to extract the average volume of an audio source. The volume is often small and needs to be adjusted to fit the control values ; the Micromap process allows to do that very easily.
Here is a video example: