Please recommend me a video editor. I am looking for a GUI application.
I haven't done any video editing recently so I do not know what the possibilities are.
Using a Linux only software is fine with me.
There is a pretty extensive number of applications which you could use to do this.
You can use PiTiVi.
PiTiVi will let you to flexibly adjust the volume of parallel audio tracks (among many other typical tasks: splice, rejoin, add a new soundtrack, fade the soundtrack in and out, fade the image in and out, etc).
On top of doing what you need (unless I understood it wrong) it is quite easy to use and comes with most linux distros.
screenshot of GUI