Not sure what you mean or are asking but.. An .m4a (wrapper) typically contains an AAC audio file that's not embedded in a video. For instance, I usually render an AAC (,m4a) file using a third-party AAC encoder, then mux it in to the MP4 which has (AVC) video.

If it does not open, you could try changing the extension to <.mp4>. If that does not work, download and install the 'K-Lite Codec Pack' which has numerous codecs. Can you run it through MediaInfo, and post the file info?