VLC and MP4 audio

[lstiefe74]

I have noticed here, that the player will play the mp4 audio files in the .m4a extension format. The issue is that some mp4/m4a format files, tend to remain silent (no audio). Does that extension (m4a) get covered by the player?
The only thing I have done is to drop composite files (both audio and video merged in to one, under the mp4 wrapper). These are most often downloaded from youtube. I have not encountered a player where both could be played simultaneously.
Would it be better to open the video and audio in an video editor, and try to merge them?

Will some one more experienced in the videos like this give me an idea what I should try to do next.