m4a is music coding format that is no where near as popular as mp3. On my android phone I have two music apps, and neither of them knows how to open an m4a file, but mp3 files play just fine.
Instead of searching for another music app that will work with m4a files, I use my Ubuntu Linux desktop to convert m4a to mp3.
The first step is to download and install SoundConverter.
sudo apt-get install soundconverter
Once installed, open up the program (it should be under Sound and Video). Open the m4a file you want to convert, click on Edit and Preferences to set your output format (mp3, etc.) and click on convert.
Super easy.
