The M4A file type is primarily associated with 'MPEG-4 Audio Layer'. Files in .M4A format are actually the audio layer of (non-video) MPEG 4 movies. M4A is slated to become the new standard for audio file compression. This format is also known as Apple Lossless, Apple Lossless Encoder, or ALE. It is a new codec designed to provide lossless encoding in less storage space.
When you import audio files directly into OnSong, they are added to your OnSong audio media library where you can link them to chord charts as backing tracks.