Audacity will give you the chance to fill out the ID3 information, so you can have the proper author, etc, listed for the MP3 file. m4b file, and then choose File -> Export. Having never converted a file that large, I'm not sure how long it will take. If not, you can have it search for them by going to Edit -Preferences (or press Ctrl+ P), go to 'Import / Export', and click the 'Find library' buttons if nothing is listed ( see here for detailed instructions)

The first time after you open Audacity, it should automatically find the LAME and FFmpeg installs. You'll also need to install the LAME MP3 encoder (to export the mp3 file) and the FFmpeg Import/Export library (to read the. otherwise, you can use the command: ffmpeg -i inFile.mp3 -c:a aac -b:a 192k outFile.m4a. I would recommend the 1.3 beta version - even though it's labeled as a beta, it's very stable. If you want to convert with the best quality and your ffmpeg have libfdkaac (Fraunhofer AAC encoder) you can use command: ffmpeg -i inFile.mp3 -c:a libfdkaac outFile.m4a. The m4b extension is just a fancy name for an AAC file.

You should be able to use Audacity to open the. I'm assuming since your file has an M4B extension, you're working with an audiobook.

Convert m4a to mp3 ffmpeg