http://mac.filesatoz.com/mp3-converter-for-mac.html This guide can help you convert M4A to MP3 on Mac os x.

1. How to Convert M4A to MP3 for Mac OS?
“I need to convert one of my songs from an m4a to an mp3 so I can
put it on my blog (WAV and MP3 sounds are supported by the site).
Does anyone know a way to do this?”
MP3 Converter for Mac is one of the most powerful M4A to MP3
Mac software to convert M4A video files from iTunes to MP3 on
Mac and also convert any kinds of audio formats, such as FLAC,
AC3, AAC, AIFF, WAV, etc. to MP3 on Mac OS X
This M4A to MP3 for Mac tool also can help you cut, split or merge
M4A audio files and then output the edited M4A audio to MP3 on
Mac with fast speed and high quality without any loss of the files.
Here, you can set the audio output parameters on Mac, such as
adjust encoder, sample rate, channels, bitrates etc.
Steps on Converting M4A to MP3 on Mac OS X
Free Download, Install and Run the M4A to MP3 Converter for Mac
program on your Mac.

2. Step 1: Add M4A files to the Mac M4A to MP3 converter
Then, Click “add files” to add your M4A files to this M4A to MP3
Mac software. Batch conversion function is supported.
Step 2: Output M4A to MP3 on Mac
Set your output format as MP3 by clicking the drop-down list of
Profiles and then choose "MP3" out of "Audio Formats".
Step 3: Convert M4A to MP3 on Mac OS
Click Convert Button to convert M4A to MP3 Mac, then the rest of
M4A to MP3 Mac task would be finished automatically by this
perfect M4A to MP3 Mac Converter. After that you can click "Find
Target" to find out where the output MP3 files are saved.

3. Now, you can play the new M4A music as MP3 on various MP3
Player.
More at:
http://tutorial.filesatoz.com/convert-m4a-to-mp3-mac.html