KB ID 0000908
Here I’m just converting one M4a file, but if you have a lot, the procedure is exactly the same.
1. Place your M4a file(s) in a folder > Open iTunes > File > Add Folder to Library > Navigate to your folder and select it.
2. File > Preferences > Import Settings > Select MP3 Encoder > (You will notice I’ve turned up the encoder settings for better quality) > OK > OK.
3. Search for the track(s) you have imported > Right Click > Create MP3 Version.
4. It will take a little while to convert. when it’s finished you will see two copies of your track(s). One will be the original M4a track and the other will be the new Mp3 version. At this point you can delete the M4a files(s) if you no longer require them.