Audio at the end of the video will automatically fade regardless of volume settings or any preset conditions. Attempting to edit volume with have no affect on this setting. However, here are some possible workarounds to solve this:
You can extend the last scene a little bit longer. This will allow your audio to run its course and let the fading happen after your audio is finished.
In addition, you can add another scene with no audio in order to let the fade run its course this way. Any scene will do. For example, a blank scene with a black background or an additional scene with a logo to finish the video.
Lastly, if you wish to cut the added extension off your video, after choosing either option 1 or 2, export the video, and use 3rd party editor to crop the video down (i.e: Quicktime).
If you have any feedback on this feature, please feel free to share on our community forum. We use this as a tool for potential future enhancements.