How do I to Convert M4A to MP3 on Mac?

One option you can try is using the free audio conversion tool, iTunes, that comes pre-installed on your Mac. Here's how to convert M4A to MP3 on Mac:

• Open iTunes on your Mac.
• Click on "iTunes" from the menu bar and then select "Preferences."
• Select the "Import Settings" button from the "General" tab.
• Choose "MP3 Encoder" from the "Import Using" drop-down menu.
• Click "OK" to save your settings.
• Select the M4A file(s) that you want to convert in iTunes.
• Click on "File" and then click on "Convert" from the menu bar.
• Select "Create MP3 Version" from the options displayed.
• Wait for iTunes to finish converting your M4A file(s) to MP3 format.
• Once the conversion process is complete, you can find your newly converted MP3 files in iTunes' Music library.

Try Shutter Encoder, VLC or ffmpeg:

ffmpeg -i input.m4a output.mp3

Hi,

See Convert a song to a different file format with the Apple Music app or iTunes for Windows – Apple Support (UK)

Jim