which Articulate 360 app to use to edit and import mp4 video/audio into Cornerstone

I have been tasked with editing mp4 videos with audio that need to be transferred into  Cornerstone.  I'm completely new to this type of process.  I have tried all the apps with Articulate 360 and none seem to work with what I need. Is there a tutorial that I could look at, that shows the process from beginning to end.  I have looked at various tutorials in Articulate 360, but they all seem to be in bit and pieces and not much information on how to use the app with external audio/video.

Ex:  There's a tutorial that shows how to import an external video and how to record audio.  If instead, I want to use the audio from an external source, it says to click on the import audio.    In my case.... the audio is already in the video but when I import it, it says no audio, how do I retrieve the audio.

Any help will be greatly appreciated.