Under Preferences - Plugins - Input is the Nulsoft MPEG4 Audio decoder installed?
If so select it and press Configure. You will probably see M4A;MP4 as the file types. You could try removing ;MP4 from here (just leave M4A) and provided MP4 is listed as a video format (I assume you mean it's on the Directshow decoder list) then try again.
Obviously rename that file back from AVI as well.
