Per design, audio at the end of a video will automatically fade out regardless of volume settings or any preset conditions. Editing the volume on the entire track will have no effect on this behavior. However, here are some possible workarounds:
1) Extend the last scene and make it longer than the audio track. This will allow the audio to run its course and let the fade happen after the audio completes.
2) Add a scene with no audio at the end of the video. Any scene will work in this case. For example, a blank scene with a black background or an additional scene with a logo to finish the video may suffice.
Note: It is possible to use a third party editor to trim the video (i.e. Quicktime) and remove the extended or blank scene.