Convert the audio to a format that is easy to edit like wav.
I dont think Elements supports m4a as audio only.
Where is this music from? A number of sources (iTunes for instance) have copy-protection built into its songs so that you can't use them in a video project. (Which is why most people buy songs for use in video projects from Amazon, which does not have this strict copy protection.)
You can also try using software like Audacity to convert the music to a WAV file and see if that helps.