You could have downloaded ConvertM4A2MP3 1.3 for Mac from our application library for free. The program belongs to Audio & Video Tools. This free software for Mac OS X is an intellectual property of UGSoft, Uwe Gohlke Software.
From the developer:
This application converts .m4a (AAC) music files to .mp3 format. This little program has a simple to use interface that lets you choose the input MP4 data by browsing your multimedia folder. You will find the start button and the conversion process will start and the program allows you to delete or to keep the original MP4 file.
You may want to check out more software for Mac, such as Aiseesoft MPEG Converter for Mac, iTunesLister or STAMPot, which might be similar to ConvertM4A2MP3.