Many multimedia converters nowadays support m4a to mp3 conversion, i.e. re-encoding MPEG-4 audio files to MP3 format. One of the most convenient tools for this is iTunes. You can convert m4a to mp3 in iTunes simply by using the Edit menu → Preferences and set import settings to MP3 Encoder. Now you just need to click by right mouse button on M4A file in the Music library and select Create MP3 Version option.
M4A audio files downloaded from iTunes Store can be protected by Apple DRM system. The simply way how to remove DRM from M4A is to burn protected audio files on audio CD and import them back as files to music library. New audio files will be DRM free.
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