Replacing Audio in video file

Is it me?
I have been trying to replace audio files in movies taken on either camera or mobile phone, of my band performing.
I can import the film which automatically gives me an audio file I can edit and export as an MP3 or Wav file. No problem so far.
However when I try to replace the audio in the video file it always comes up with invalid or no supported format when I select the video file on hard drive. As it is the same file as I was able to import the same file in the first place, I cannot see the problem.
Any ideas?
Mac 20009 2.66hz Intel Core duo, 4gb, OS X 10.9.5